never crossed that line. I have done other guy's gfs when I was younger. the closest I ever came was a nite when the gal kind of picked me up in a bar. we went to her place, kissing, feeling each other up. she pulls my cock out, and then explains that she is getting married in a week. doesn't think she should be doing this, but wants a big cock, a different cock before getting married. she backs out completely. I walk home.
The next nite, same bar, same chick. she has changed her mind, she says. let's go now, let's fuck. so after a couple more drinks we go to her place, and she changes her mind back in forth. at one point she is calling her sister, her sister is telling her to fuck me. I vaguely knew the sister. anyway, we finally fuck. after pumping the hell out of her, and thinking 'she is not gonna cum, cuz of her head'. she just ramps up and cums all over, and I cum. she's laying there moaning, quaking, & shaking. tells me to take her car and leave, come back in the morning. so I'm out driving a Z28, I check the registration. it's in her fiance's name. I fucking his woman and driving his car. I come back the next morning, and fuck the crap out of her for a few hours. that's it, we're done. she gets married the next weekend. her sister tells me thanks a few weeks later, and also tells me that the new husband knows what happened but doesn't know who the guy was. he eventually figured it out yrs later, but didn't do anything. shortly after, they got divorced. and in 5 yrs she went from pretty foxy to pretty ugly.