Or, a (brief) secular investigation of a religious ethic.
Learning from a given code of conduct does not mean obliging oneself to all of it, nor does it mean that out of either one's own identities or out of respect for borders those of another have erected around themselves must we not learn from them. Cultural appropriation can itself be a positive cultural norm, and there is often a lot that one can learn from the way others do things. I disclose that I am neither a Torah scholar nor should I be taken as necessarily authoritative on the various forms of normative Judaism or Islam - I am just moderately well informed and interested.
Shomer Negiah is a term attached to normative observant Judaism (and, under different terms, in a parallel form in Islam), whereby touching is restricted between nonmarried people of the opposite gender who would be eligible for intimacy (close kin, for example, are not fully bound by the custom - more on this in a bit). Stepping beyond the customary reasons for obeying such things (because it is tradition or interpreted from holy works - neither of them things seculars should consider obligatory or positive), we can instead look to the societal problems observance of it solves (an interpretation practice presumably central to Reconstructionist Judaism? I have not read enough Mordechai Kaplan..) and consider its application and variants.
Both Judaism and Islam, as I understand, constrain the notion to people who would be eligible for intimacy (meaning marriage) - people raised together as kin do not (under some interpretations) qualify for these restrictions (meaning kissing one's uncle, hugging a parent, etc would be considered ok). We understand that sexual attraction is rare among people who have blood relations or who were raised together - something about human psychology tends to make that a non-issue (perhaps solvable through some group-selection effect in Ev Psych). The focus of the practice, this suggests, is on romantic-relationship behaviour. Presuming a monogamous relationship exists (which in normative Abrahamic religions is roughly what is steered for (although the steering is theoretically not quite that narrow)), with people not acting with some reasonably similar custom, we find a potential social harm - relationships (marriage, actually, but I'm being slightly self-serving with scope here for purposes which will later become clear) become ill-defined and jealousy/feelings of betrayal (which are presumably hard-wired into us but alterable to a certain extent) become likely, with no well-defined solution (people may, of course, negotiate with their partner, but for some this is distasteful, and the lack of a norm to gently-or-roughly-shove people towards a roughly single set of behaviour, good matches between people become harder because of another potentially disqualifying foundation for relationships, people are pushed to ignore primal jealousy which can lead to all sorts of ugliness (youth are particularly unlikely to understand themselves and become hurt), and societal fitness may decline from both of these (this is not meant as a conclusion/argument so much as an observation). Shomer Negiah steers society away from these problems by providing a code of behaviour that protects monogamous relationships (marriages, in Judaist/Islamic implementation) from interference from friendships and things-between-friendship-and-relationsh
Shomer Negiah (and its Islamic cousins) are not the only type of norms that could serve this social needs. Further, Shomer Negiah is particularly about marriage - while marriage does not have to be the for-life partnership it is today (and some forms of Shi'a Islam permit short-term marriages that may in some ways resemble the monogamous dating common in Western nations), we can understand it as being intended to, as above, protect the relationship and prevent instinctual jealousy from causing strife by establishing clear bounds (although the specifics of this vary reasonably broadly across communities) on relations with others. Is this necessarily only useful for marriage? Stepping outside of only considering Halakah/Sharia, we find our concept of dating - presumably some form of "going steady" (pardon my possibly antiquated terms) has these concerns as well - being monogamous with a date, even at a stage when one does not necessarily expect it to last forever, is somethting which many people expect of each other, and there are potential issues over intimacy with others that are largely similar to those that married couples face. Speaking personally, marriage to me is just a recognition of a relationship that both parties expect to be durable and for which they would make considerable sacrifices, and I reject the "no-sex-before-marriage" credo as unwise (sexual compatibility is as important as other types of compatibility in a life partner - one should explore every aspect of a partner/the relationship that might be important before expressing a durable commitment, so much as is possible). Even from that perspective (my perspective is *a* modern one, but by no means the only), something like Shomer Negiah seems like a possible starting point for how I would expect my partner to behave around others, and how they should expect me to - limits on their intimacy with others would protect the relationship and prevent what would possibly be a violent (or at least very very loud) response to their violation. Thus, the norms, repurposed so that what is reserved to marriage is instead reserved to monogamous relationships, and so that the norms don't apply at all to those not in a monogamous relationship, are useful.
Shomer Negiah comes in many forms, and different streams within the various forms of Judaism/Islam have very different takes. Some literally prohibit all touching, some prohibit touching below the belt, some permit handshakes, etc. What's most important here is that these norms are established outside the specifics of a situation, and that people are nudged towards having roughly the same norms. The first is quite important because many people are easily driven, in the heat of the moment, into behaviour that they would regret in retrospect and would consider unwise if considered beforehand. I hold that this suggests that having one's value-system worked out beforehand is usually wiser than making spur-of-the-monent decisions (their presence as strong norms, and the emotions we tend to attach to them as such, still give us the flexibility to modify them for exceptional situations, which should be fairly rare in value matters like this). The second matter is important because it is exceptionally undesirable to have matters of this import always be a potential full disqualifier for relationships, and the "invisible hand" (not an economic one, but rather one born of the dynamics of the system) pushing people towards the most permissive relationships ignores the harms that we're talking about. Also, too much flexibility makes it difficult for a community to protect the relationships in its midst (e.g. by shunning those who go against the expected norms of a relationship - those who cheat, for example, should ideally find it very difficult to date again and face a great social cost until/unless they make a good case for having had a change of heart).
Another modern complication would be the existence of non-heterosexual people - given that I don't consider heterosexuality to be the only permissible way to be (nor even preferable over bisexuality or homosexuality), I would suggest that a secular shomer negiah should either be dependent on the known sexuality of the people involved (e.g. a bisexual person should have more things to be careful with than a homosexual or heterosexual) or apply regardless of gender. Further, the social norms, since I've suggested they should no longer prohibit extramarital contact, should be specified to suggest that the obligation not to interfere with monogamous relationships should extend to third parties as well (that is, if impropriety is initiated by a person not party to the relationship, that is still not considered societally acceptable for that third party, regardless of how the person part of the relationship reacts).
For what it's worth, even given all these nuances, the norms/bounds I think are important are reasonably permissive by most religious standards, largely derived from a modern interpretation of "what is sexual/sensual or reasily transitions into either" and fitting into a broader framework (similar-but-different in the same way to parallel frameworks in Modern Orthodoxy/western Islam) of appropriate social closeness given relationships.
The very broadest intuition that covers all of this is: When in a monogamous relationship, having boundaries for what one would and would not do with others, and keeping those in mind in real life, is a good way to avoid a lot of trouble with consequences. When these boundaries are widely accepted, society works smoothly. Those not in a monogamous relationship should act with as much care when relating to those who are as if they were in the relationship. Also, religion-as-a-set-of-customs is not a bad place to look for potential approaches-or-inspiration-for-them for modern problems. Seculars should be willing to toss things out and rework others for modern norms and values (and no, the Ten Commandments is not, despite what some people say, as a whole a good foundation for modern secular norms), but understanding other approaches to a problem can often shed some light on subtleties of the problem as a whole - a good secular thinker should be well-read on religious matters.
I further assert (but won't explain right now) outside of the bounds of this analysis that it is emotionally horrible to ever place someone in a situation where they are forced to choose between being broken up with or accept effective or real nonexclusivity. By the way I look at things, that's being a real jerk, far worse than simply ending the relationship - it's different if it's mutually arrived at (understanding mutuality as in "we-both-want-this" rather than someone-being-pushed-into-it with that threat).
People who are particularly interested should find the terms (which I once looked up) for the equivalent concepts in Islam - I believe there are words equivalent for "Shomer Negiah" as well as "being of relations that establish people to be ineligible for itimacy".
Tablet: ordered. Also, GIMP 2.6 came out! I hope it doesn't look/feel more like Photoshop (which IMO has a worse interface - far, far worse on windows than OSX though).