I think it sometimes has to do with an even deeper-seated fear: that of being stupid. I've noticed in my world that a lot of people (especially, though not exclusively men) are worried about 'being wrong' or 'being duped' in situations that are even clearly to their benefit.
The men I've known who are worried about this kind of thing (getting their basket grabbed, or getting fucked-up-the-ass [accidently?]) are conditioned to believe: 'scoring with the chicks' is the desireable position. 'Scoring with the dudes' is a distant 'second place' ... so there's the calculus that says: if you do it with a guy, you're 'losing' or at least 'missing out' ...
But really, it's less about actual sex than it is about being duped. Which of course begs the question: how stupid do you have to be to be worried about 'losing out' by getting your basket grabbed :-)
/jordan