I think at first its the deal of wanting what you can't have. he could be a cool, great guy (or just an average guy, or a loser in some girls cases) and on top of that, he's unavailable. you want him even more now, if you ever wanted him at all, that is. also, he's committed to another girl and made one of the biggest symbolic moves for commitment in our culture. you think, oh that's a guy who is serious, and there must be something great if another woman chose to marry him. he's definitely boyfriend material, husband material...maybe we can be together. Its a thrill chasing what you can't have, even if you don't really want the same commitment from him, which you usually dont. Sometimes you do, though.
Also, if he does flirt or cheat, there's definitely a thrill there. definitely. its forbidden lust, sometimes love. its hot, its dirty. you're two little dirty sex monkeys doing what society, his wife, the priest, rabbi, etc. told you not to do. very much like children who want to break the rules even when they have been taught that its wrong or that there will be consequences. in that moment, you don't care. there's an adrenaline rush, some intoxicating feeling of rebellion in something as simple as sex here. what could have just been great sex between two sexy people on any other day, in any other situation, is now an explosion of flavor, you're dipping your fingers, your entire f***ing hand into the sauce they told you was off limits, poisonous, or "only for dessert, kids." its an unbelievably delicious flavor to have what isn't yours, what you're not supposed to have, and it's sometimes mixed with an after taste of guilt.
lmao. well, see, I've been "the other woman" more times than I care to admit. its really that good to people like me. for other types of people, not so much.