"Keeping the vein of that analogy though Fubar, lice-picking and sex are ways that primates bond. They don't do those things with strangers from another primate group.

Hugging and sex are ways humans bond."

Well that's my point. At some point humans decided that lice-picking wasn't bonding, but was utilitarian, and so it was acceptable just to pay someone to do it for you.

In many other primate communities, animals are promiscuous, and sexually active couples or groups don't always bond for life.

Lest we get in to an argument that 'humans aren't animals' my point is just that if sex is so shameful and prostitution so dirty, why is it 'the oldest profession'? In my mind, sex isn't shameful, nor is promiscuity. It may be socially unacceptable, but I think that's for cultural reasons, not for inherent behavioural reasons.

There are plenty of taboos that are now acceptable and offered as a service/commodity:

Organ donations - technically it's illegal, but a black market does exist for human body parts.
Surrogacy - some women are happy enough to pay for another woman to carry their child to term
Wet nurses - allowing another woman to breastfeed your baby isn't considered gross by everyone
Counselling - opening up to a therapist and telling her/him your deepest most shameful secrests isn't, oddly, shameful to most people

Some cultures would regard the above as shameful, but other cultures find them acceptable. I think sex is no different - the associated shame is created by social values and little else.