When I was a freshman in college, I did it with a girl who was a senior in high school & engaged to a guy in the Navy. I asked her out, and she didn't mention that she was engaged until we were on the date. She came on to me, apparently because she was horny as hell, and I was glad to cooperate because: (1) she was very hot looking; (2) she was one of the best kissers ever; and (3) I was an 18 year old guy. Over the course of 4 hours, I had 3 orgasms and she had 4. I could have fallen for her, but she gently let me know that she loved the guy, had every intention of marrying him, and just really needed some lovin'. She did indeed get married about 6 months later, and to the best of my knowledge she never cheated on the guy again. That was 44 years ago, I'm told they're still married, and I've never felt guilty about it for a second.
When I was in my early thirties (by this time I'd been married for over 10 years), I had an affair with an engaged co-worker. She also came on to me, and I was eager to give her a try. The affair lasted a little over a year. The sex was awesome; she knew how to fire me up by telling me how big my cock was and how good it felt. Sometimes after I had come inside her, she'd roll me over and lick all the juices off my cock, then gently suck it and tell me how good it tasted. The affair was great for a while, because we were both just looking for physical satisfaction. However, it turned into a nightmare when she broke up with her fiancee and decided that she wantred a serious relationship with me. She was emotionally unstable and constantly demanded attention; if she didn't get as much atttention as she wanted, she'd threaten to tell my wife everything. I was fortunate to extricate myself from this mess when she found another guy who would try harder to satisfy her neediness.
Since then, I've gotten almost all my extra-curricular sex from massage parlor girls. In the long run, the massage parlor route probably costs less money than an affair, and certainly takes less time & hassle. Also, the parlor girls I've frequented have a lot more talent.