I haven't read all the replies yet, so forgive me if this has been mentioned.
What about straight guys who hit on gay guys for a little "servicing"?
One of my best friends from high school came to visit me when I moved to Toronto (from our small town in Northern Ontario). He didn't know I was gay...but he soon found out as I shared a small bachelor apartment with my lover at the time.
The night he visited, the three of us got stoned. My lover fell asleep. A little while later, my friend and I hit the sack. I slept in between the two guys. Suddenly I felt something nudging me in the back. I reached around and grabbed what had been my biggest fantasy in high school. I turned to my friend and said, "Get in the bathroom NOW!"
We went to the bathroom and I immediately went to work on him. This wasn't easy because based on what I knew about my cock size (8") I could tell he was at least 3" longer than me. He also had a girth that would stretch even the most talented mouths (he had one of those dicks that look like it could iron clothes if you got it hot enough...if you gay guys have seen the kid named "Horse Hung" on seancody.com, you know what I mean).
Anyway...I worked on him for close to a half hour. When he finished (drenching me). He thanked me and gave me a hug. We went to sleep.
A repeat of this incident happened a couple of years later. He never reciprocated.
He confided in me that sex was difficult with women. He hurt them vaginally, and only a few would attempt to do him orally. I really felt/feel sorry for the guy (we're still friends, but live hundreds of miles apart).
I NEVER come onto straight guys! Why bother? But in these two cases, I was more than happy to service a straight guy who came onto me. 1) Because I was always curious about the huge bulge he had in his pants and 2) Because he was a great guy, who had no hangups about me being a fag!