It depends on how broadly you view "transactional". I highly doubt the original poster intended it to mean any relationship where both parties derive some benefit. The context was the question of whether to buy the services of a prostitute, and the poster appeared to be distinguishing sex for money from each party having sex for pleasure.

In light of that, suppose we begin with a narrower view and say that a transaction requires each party to exchange some kind of valuable commodity or render a service, then much friendly interaction ceases to be transactional. In general, allotting a certain time period for fun activities is a trade-off you make with yourself. If that time happens to be spent with friends who are all there to have a good time, then no one is really engaging in this kind of transaction with anyone else. Everyone benefits, but there's no real exchange of valuables.

Under this view, a transactional approach to a relationship would be one where every interaction is viewed as an exchange. Consider the gold-digger approach, for example.

I think this approach gives a context where the original statement makes a lot more sense. I'm sure one can find other interpretations of "transactional" that also work.