Well I think its mostly a matter of attraction. If you're not into guys...or their cocks, then there'd be no desire to suck dick.
But I guess that'd be obvious.
If you're talking about a threesome scenario or what have you, I think a lot of it may have to do more with the stigma against homosexuality that still exists among a lot of straight men.
There's this strange perception that women can dabble and experiment while remaining straight, yet men are either gay or bi the moment they cross that boundary.
Its strange, but I think that might be what you're talking about.
I for one identify as straight, in that I don't find men attractive at all (I seriously don't know what women see in us)...but I'd be willing to suck some mad cock in the right situation.
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