As a set of rules for exonomic relationships in society, we consider systemic harm and force inherent in capitalism.
Under no circumstances would we permit or consider moral someone holding over someone hungry an offer to exchange food for sex. This would amount to rape. Beyond that, we consider there to be a positive obligation for anyone with their needs met to abstain from further societal provilege until the basic needs of others are met. This is because wealth and property are society's perks to shape behaviour, and society can and should never allocate privileges designed to incentivise one person come at the expense of another's basic needs.
I am willing to accept that some differences in material privilege may be acceptable in society when it serves legitimate societal interests.
Beyond basic needs, where every person has a moral right to have them met, we are willing to make demands of people for their reasonable needs to be met. Some of this is practical; beyond the emergency needs above, society still must be solvent in its use of resources, and most kinds of labour (even if orthodox marxists would call this slave labour) can be shaped in ways as to make them not terribly dehumanising (labour protections, etc).
Sex remains meaningful in human relationships in a way other kinds of potential paid action are not. The kinds of consent and pressure that would be appropriate for a grill-order cook are not appropriate for sex.
We would like to build a society with strong, reliable, and understood safety nets for people, under socialism or capitalism. Even with such systems, we still reject additional privilege for sex work, because we recognise that the concept of need is a fluid one and things that people are strongly accustomed to (from reasonable housing to computers) offered for sex still come from an unacceptable topic of pressure. Even if we accept wage-slavery, we should reject sexual-slavery.