The best I can recall was when my (now) wife met and began dating.
She was hot, in heat, and had lots of men (and boys!) sniffing around. We were fairly casual at first, enjoyed great sex together, and were both still free to date and/or enjoy sex with others.
Several weeks later, as we were becoming more serious and exclusive to each other, she decided to fuck a (very!) young co-worker who had been hitting on her, and shared this news with me. She could see him a day or two during the week, and we saw each other on the weekends.
Although I knew she was doing it, she was still reluctant to share details, but loved'teasing' me about it.
She came over to my apartment one weekend, and as we later began to have foreplay in bed, I went down on her. Nothing unusual as I loved looking at, touching, and eating her pussy out. What was unusual was I noticed right away her lips were quite stretched and hanging a bit, and somewhat swollen.
I started licking her pussy as deeply as I could, and told her I tasted dick and asked what she had been up to. She said that the young guy had been over and that she had been "playing with him all afternoon".
So I spent the next while sucking and fucking her pussy while she told me how she seduced him, how they fucked...all the wonderful details.
She was cumming and squirting like a champ, which just added fuel to the fire. We were able to repeat it several times as each new detail was inspiring and motivating.
Easily the hottest sex I've ever enjoyed! Everything came together that night in perfect harmony.