I used to have a lot of hangups about getting bred. Not because I was worried about STDs (I only had the one fuckbuddy and I knew he was clean), but cause I felt like I felt like it was really humiliating.
I already had a lot of insecurity about my sexuality/masculinity. The only reason I was willing to take it up the ass in the first place cause me and my buddy were doing the whole "we're not gay, we're just 'trading favors' to help get each other off" bullshit. We both played reluctant about sucking dick or getting fucked, and acted like we were only putting up with it to get our turns on top. But the truth was, I was getting increasingly turned on by it.
My friend picked up on that, and started teasing me about it and pushing my boundaries. We normally used rubbers for anal, but he eventually goaded me into letting him put it in bare. He fed me the standard "just the tip" sweet talk, but soon he was balls-deep in me, swearing up and down that he'd pull out before he came. Of course that was BS too... when he came he pinned me down and bred the fuck outta me.
Afterwards I acted pissed, and he made half-assed apologies about "getting carried away." But it was actually a huge fucking turn-on for me, and he fucking knew it. That was the start of a permanent shift in our relationship from an "equal trade" kind of arrangement to a top/bottom relationship, with me in the submissive role. I'd usually act like I didn't want it, but he knew that was just a bullshit act so that I'd feel less gay about giving in to him.