Tom, I initally had a similar reaction about the word "reason." I'd have found it more satisfying if the study had explored the things that triggered people's desire and the conditions in which they found they could act sexually on their desires. But the more I thought about it the more comfortable I got. For one thing, as you point out people do have sex even when they don't want to or when they think they shouldn't even though they do want to.
But also, remember that the study was based self-reporting, and people do often think of what they do in terms of the reasons behind their actions. I think your analogy doesn't quite work. If you put a plate of food in front of me, I might not eat even if I'm hungry because I might not like the food. My reason? I know I can get food I like without too much trouble. (This is based on a certain amount of privilege, to be sure!)
On the other hand, even if I'm not hungry, putting food in front of me that smells fantastic or that I get pleasure from eating might be reason enough for me to eat despite my lack of hunger. I've certainly eaten enough chocolate when not really hungry to bear that out, I think. I think the study is problematic for lots of reasons (there's that word again!) and the word "reason" is only one of them. (We often don't know all the reasons that we do what we do.) But given the fact that the study was entirely based on self-reporting, I think the word "reason" is the least of the problems.
...because public space really matters!
Elizabeth