Years ago I has a sex only gal friend I was seeing. I was 22 she was 20. Ugly nerd type with a rockin body. She had a boyfriend. She would stop by my place one or two times a week for sex. No head games, no drama just sex and it was good sex. It worked one of two ways, she'd call around 8, ask if I was busy and come over. Or she'd call (from his place) around 11 and come over. If it was a late call she was at his place having sex then call me. I knew about him, he didn't know about me. She was very open about what they did on their date, dinner, movie, whatever....then how many rounds of sex they did. She was really cool about it admiting she was still horny after she left him.
We hooked up many times over a few months. At first I didn't figure out the early or late call. Then one night she came over about 11:30 saying how horny she was. After the usual 5 minutes of small talk she let it slip she was out with her boyfriend, no biggie to me until we started to get "busy" and it all clicked in my mind. Yea I was getting his sloppy seconds but damn it was good sex. After we got done she came completely clean saying she loved him but really liked the sex we were having too.
Hell yea I could tell the difference when I'd slide into her.