In anthropology and other social sciences, sex in return for money, goods, or favors.
For example, in Sub Saharan Africa, many teen-aged girls resort to sex with "sugar daddies" -- men in their 30s and 40s -- in order to pay for their secondary schooling.
Within the Man2Man Alliance, however, and particularly in the writing of Mart Finn, the term transactional refers to sex with a commercial aspect, even where no money changes hands.
As Mart says
In my work I do not associate any exchange of money or goods for sex when I talk about the experience being a 'transactional'one. By transactional I mean a kind of sex, whether it be casual or in a relationship, where one party or usually both are psychically distanced from each other. They are 'doing sex together' ... for themselves.
The concept is inextricably linked to the commercialisation of gay male attitudes to sex and often life. Sex is just another activity. To be compared and evaluated against previous experience and those of our peers and dominant subcultural norms. If our expectations and an ideosyncratic paradigm of desires are fixed as we engage, embrace ... come to our lovers, then we are asking them to fulfill something for US. We fully intend reciprocity ... if they can stimulate it from us.
I understand what the anthropologists are on about ... transactional ... indicates money goods or favours. Yet we know there are many other forms of transaction which are not so overt ... the motivation of modern western men and women when they engage in sex can be one of them.
See Mart's essay, CockToCock, for more about the difference between interactional and transactional sex.
See also Interactional sex.