It's only happened twice for me and both were with the same straight friend. I wasn't sure he was straight before it happened and even less so afterwards, but time has proven him out and I'm now pretty convinced. About 5 years later and he's a guy that could be out if he wanted (no family pressure, lots of open friends, both straight and gay), he's hot and could have lots of guys, but he's been with women and apparently very happy.
Anyway, to the stories. The first time was odd. I was with him and my gf at the time (now my wife). We were partying all night and he was crashing with us. We got home, kept drinking and hanging out and then it somehow turned into a drinking game. The drinking game turned into a game that also involved getting naked. We all got very, very drunk and ended up naked. At this point it seemed like it was heading towards some very interesting things - 3some, etc. He and she were making out a lot, which got me very worked up, and she and I were making out a lot, and he was hard. Suddenly, the alcohol took its toll and she started to get sick. She ran into the bathroom (I followed to take care of her) and then she passed out. I moved her into bed and then came back out to see how he was, say goodnight, or whatever. When I came out of the bedroom, he was standing in the other room (it was a small apartment) watching me. He was still hard. I walked up to him, he was still really drunk and even hornier after the teasing with her earlier. He put my hand on his cock (knowing that I was bi) and had me stroking him. This got me turned on, so I pulled him into the living room, sat on a chair in front of him and sucked him for awhile in that position. Then he pulled away, sat down on the couch and asked me to finish him. So, I got onto my knees, kept sucking, he asked if I wanted warning, I said yes, then stayed on and let him finish in my mouth. It wasn't too weird afterwards, he thanked me, and then we went to bed.
The second time was maybe 6-8 months later. My gf (now wife) and I had been fighting. It was a weird time for us where we were deciding if we were going to commit for real or what. We were kinda in an open relationship. We had had a fight that night, she went out to fuck someone else (which was fairly normal for both at the time), and I stayed home pissed off and drinking. I was getting pretty drunk when the same friend called, visiting town again, to ask if I wanted to go out. I said no thanks, told him what kind of night it had been, and that I was staying home. He asked if I would like to talk or whatever and I was indifferent, but he stopped by to see how I was. When he got there I was very drunk and had porn on the TV - straight porn cause it was pay-per-view and that's all that's offered in that form around here. He sat down, we talked for awhile, he tried to ignore the porn, but wasn't very good at it. I was pretty depressed and kept talking about going out to find a stupid hookup, he was talking me out of it because I wasn't in a good place for it, etc. Finally I said fuck it, I'm going to go find someone to blow (yeah, I had certain hopes for where things would go from there). Finally the porn and my end of the conversation got to him, he was hard as hell (obviously) and he said that I shouldn't go out and be unsafe, instead I could blow him cause he was horny and it would be safer. I, very quickly, got his pants down and sucked him off again. This time I pulled off for him to cum on my face. I cleaned up, thanked him (haha) and then we chatted for awhile before he went out to meet his other friends in town.
We are still friends now, but haven't seen each other in ages - just kept in touch online. I doubt it will ever happen again, I know he was pretty uncomfortable after the last time, but it was really nice while it happened. He was super hot and it's still something that I jerk-off about today.