I fucked my best friend's wife, before they got together, and later married, and he knows about it.
We have known her before they became a thing, she used to hang out at this place we all visited. Not beautiful, but kind of cute, and what set her aside from others were huge natural tits, combined with pretty slim figure.
One night, there was a big crowd, everyone got hammered, and I took her home. He wasn't there that night, so tomorrow morning I bragged about my exploits with her. He was very excited, asked me if I did her raw - I did, and if I came in her - I didn't.
I wanted to do this again, but she wasn't interested. Few months later, it was his turn, similar scenario, only this time I was there, to see him take her away. He called me the next morning to brag.
After that ,it became somewhat of a thing between them, and eventually, they started dating, which ended in marriage.
Is it still awkward, when we meet? No. Was it? Oh man, it almost ended our friendship, but we somehow got through.
We never talk about it, nor about anything sexual, any more.