So here’s a problem that arises from time to time. Your buddy’s dating some hottie. Now because you’ve always acted in a fun way around here and could be comfortably playful without hitting on her… because she’s you friend’s girl… she becomes attracted to you and starts to flirt with you.

Of course you like it and of course you’re attracted to her too. That’s what happens when you’ve got a hot girl flirting with you… especially one that you can’t have.
So what do you do. We know what you want to do, you want to wait until you have an opportunity where it’s just you and her, some alcohol is involved, judgement gets skewed, and you end up ripping each others’ clothes off when the passion takes over and no one will ever know.
Like I said, we know what you want to do… but the question is what you should do.
 You may not expect me to say this, but sometimes you should fuck her. I know… awesome. But the thing is, you always have to put your friend’s feelings first; kind of like having a baby.

The simplest way of accomplishing this is to talk to him. Say very bluntly, “I’m attracted to X she flirts with me… I kinda want to fuck her. What would you be cool with me doing?” Get down into the nitty gritty here. Maybe he doesn’t care if you make out with her but doesn’t want you getting a beejer. Maybe he’s cool with you getting head but doesn’t want you fucking her. Maybe he wants her to be cool with him seeing other people, so he’ll encourage you to fuck here. Maybe he actually has feelings for her and wouldn’t be cool with you doing anything. Whatever he says, stick to it.
The most important thing here is that your friend is more important than any passionate night. There’s a million girls out there. Plus, you should always be good to your friends anyway, a man who is respected by other men gets more women. 

The same rule holds with ex gfs. Talk to him. There’s usually one or two girls in a mans’ life who he would be devastated by if a friend fucked her. I have mine. Stay as far away from that one as possible.

Basic rule of thumb: Don’t be that guy. No-one likes him. You may have one amazing night with a girl, but in the process you’ll lose/damage a friendship and lower your value to every woman who finds out. 
Now if your buddy’s cool with it…