Oh there are a few at least. But there are definitely a top two.

The most frequently fantasized about would have to be using a strap-on with another woman. Every experience I've had with women has managed to happen without one. Normally they haven't even involved toys at all. Just good, old-fashioned hands-on (and mouths-on) work.

And I find it interesting that when I do have these fantasies I'm normally the one wearing it. I prefer more aggressive and androgynous women more often (but not always). I think the desire to use it partially comes from wanting to please my partner in every way that I can. A variation on this does occur where I'm being taken from behind by someone of the male persuasion while this happens.

The second fantasy would have to be sex with two men. I've had mff threesomes before and while group sex doesn't seem to be my favorite thing in the world (I tend to feel overwhelmed) that doesn't take away my desire to please two men at the same time. Really the idea of pleasuring them in just about any configuration sounds wonderful.

Honestly, at the moment the second scenario seems more probable than the first as I have more male prospects than female prospects. But, if i had to make a choice between them I'd probably jump at, or on, or in the first fantasy over the second.