The first time I was ever in love, really really in love, it didn't last. He left me. The pain was so different from anything I'd experienced before. I hoped never to feel like that again. By the next day, I was still very depressed, but I was also very sexually frustrated. Very. It remained that way for a few weeks. Someone at school began hitting on me, and while I didn't even have much interest in getting to know him, I thought I might experiment with physical satisfaction without emotional closeness, something I'd never done before. I was very close friends with my lovers to that point. I made it clear that I was messed up from heartache, and that he was not to get attached to me, but that I was interested in sex, if he felt confident he could perform well. I told him I was used to partners a little older, with experience, and that I wasn't emotionally prepared to deal with a fumbling virgin, and I didn't want a new boyfriend. I was exactly as cold and blunt as that. He said he was okay with those terms. The sex was terrible, probably because I didn't realize how much pressure that kind of frankness would put on a young man, because... I just didn't know better yet. At the time I thought maybe I just didn't like sex without love. I resolved to wait for love.
A month later I met a guy at a sci-fi/fantasy convention. He was obnoxious, and he followed me around like a lost puppy. Creepy. He asked my friends about me. He described my cosplay to strangers and asked around who had seen me. I'm really good at losing a tail in a crowd, but every time he lost track of me, he chased me down harder. I have no idea how, but over the next few months we would become friends, then lovers, then best friends. Our love was the envy of all who one us. Strangers would tell us we we're sweet, and ask if it was our first date, even when we were years into our relationship. They'd tell us we gave them faith in romance. This was the first really big, deep, all-consuming love of my life. We became part of each other. We were Team Us. Despite all the horrible betrayals in the last year or so of the doomed relationship, we remain close, twenty years after the day we met as of last month, I think, after a lengthy cooling off period. I'd never fuck him or anything, but, no one knows either of us the way we know each other, including the people we married.
We broke up on a Tuesday. By Thursday I was talking to a new man. By the following Wednesday I was under him, gripping him with my whole body, screaming his name like I needed an exorcist. That time, it was satisfying, and I learned that I just didn't like the dude I first tried casual sex with. I didn't need to be in love, I just needed to be familiar and like him, he needed a great personality like everyone else I keep close. Oh, and no relaxed standards as far as appearance. High attraction only.
All of this is to say, a week before I fucked that dude I did something I had never dine before. I had never known of him. I never cheated. I just came from a relationship in which sex happened an average of 8 times a week. Amazing sex. Mind-blowing. And I was just as unhappy losing that sex as I was losing my cherished love, but he was abusing my trust, and he had to go. I couldn't replace that, but I could replace the sex, maybe, I thought. A week later, in December of 2004, I started fucking a dude I fucked on and off until he became a believer of flat Earth theory last year. I'm sorry, but I cannot fuck that. No. Jeebus. Anyway. A week, and he went from stranger to fuck buddy to friends-with-benefits for the next 13 years.
Ever since I met that guy I was totally sold on the benefits of casual sex. I've had heaps of fantastic and interesting experience. I've loved a few of my casual partners. I've broken a few hearts. I've experienced disappointment. But overall, it has been really good for me. So, whenever I have a breakup, I get laid as soon as possible. If I already know someone attractive, especially if they want to listen to me whine about my sadness, I just see if they want to get it on, unless there is good reason not to. I do this whether I caused the breakup or not. Now, a good reason not to includes if that dude is friends with my ex. I always go outside the circle. In fact I recently, after 13 years without contact, ran into my ex's best friend. I never noticed how handsome he was before. Attraction seemed mutual, but neither of us was interested in going there. We are off-limits to each other. You know. Because we're good people.
Your ex and best friend are simply not good people. That doesn't mean she cheated. Just as you saw her show up the night you saw, you'd have seen it before too.