It depends I suppose on what you mean by "transaction". As I understand it, transactional analysis is a school of psychology that posits that all human interactions are "transactional" in some sense - i.e. they involve exchanging something for something else. Even a "Cold out today...Yes, wonder when spring will finally arrive" conversation in the supermarket becomes a transaction on this understanding, because it's a sort of exchange of recognition of shared humanity.
But that's not (AFAIK) the only psychological theory of human interactions, nor does it seem to me to be the sense in which MRAs mean the word in any case. They are thinking specifically of financial exchange within a capitalist society. So a married woman barters sex for her husband's earning power and capacity to buy her a nice semi in the suburbs, in their view.
I suppose I see lots of different models of sexual behaviour going on here:
The financial transaction view (MRA)
The competitive sports view (there's an imaginary panel of judges going to hold up marks for artistic impression and technical merit at the end, you must tick off a certain number of positions from the karma sutra of a certain level of technical difficulty to even be in the game).
The transactional analysis view (you give me pleasure, I give you pleasure view - I've been in relationships which have ended up like this and they always seem pretty unsatisfactory)
What I've heard called the "jam session" view - sex is like playing music, something you do with another person you like and have chemistry with (or shared musical tastes). IME this is the sort of sex I like best.
So I guess I don't buy into the world view that says all interaction is transactional, so I think that arguing that even sex centred round mutual pleasure is transactional is stretching the meaning of "transaction" to breaking point.