First off, I'm gonna ignore the stereotyped generalization you made in your OP and try to answer your queston... *cough*douche*cough*
So if you meet a straight guy that gets your interest and he seems like the type of guy that could possibly take the dick. You have to play to his emotions, first get to know him, as best as you can, get him to trust you in a way that he can come to you and tell you things with the thought in mind that you wouldn't tell anyone.
Stroke his... ego... be in his corner if something happens. Basically be his buddy, close buddy. Occasionally talk about sex, get him to talk about his previous sexual encounters. Talk about your past sexual experiences but be vague, don't let on too much about you sleeping with me. Even though he may know that you're gay, it's best when talking about sex to keep his mind thinking straight sex.
Once the conversations become comfortable talking about sexual experiences, start again with stroking his ego, but now telling him basically how much of a stud he is, subtly though, once he lets his guard down a little more with you, start to slightly give him the wink and nod that, if he ever needs "help" that you'll be there to "help" him, given he doesn't have a woman to be there for him.
Tell him this jokingly but be serious at the same time. What you're doing is putting in his mind that you're trustworthy and can keep secrets, you're his buddy and you have his best interest at heart and that if he's in a desperate situation, you'll be there to "help" him with his tension.
Now all of this can take some time, but with patience, it's bound to work, if you play your cards just right. Trust me, I've done this to about 4 of my straight buddies, all of them in the military.
This goes without saying but the straight guy in question must be an open guy to new things, if he's your typical conservative douche bag straight guy, then it probably won't work, but this guy has to be gay friendly and comfortable with himself... basically the guys who embrace Man-love Thursdays...