Not quite cuckold but...
Years ago I had a sex only friend. She'd call me around 7, bring pizza or Chinese, we'd eat, watch TV then fuck for two hours.
She had a boyfriend she said she loved. I was 22 she was 23, not very good looking but had a rock solid body.
Or...she'd call me around 10-11 from his apartment (this was way befor cell phones). They would go out then have sex then she'd call me. He didn't know about her and I. Yes I was getting his seconds, she got off big time on that aspect.
She started to stop by my place before their date sometimes. She was pretty open with me about their sex fun. She freely admitted to me how horny she was after she had set with...him or me...She told me he loved the sex they had after she'd been with me.
Is that cuckold ?