In almost all of my relationships, I've regretting not getting one last goodbye lay, but in reality, it doesn't work like that (h00man's case being an exception).
But I see why he did what he did. If she told me she had recently been with someone else right before we were getting intimate, I wouldn't be in the mood to get close to her. Not because I would think less of her for sleeping around, I honestly wouldn't, but it would make me question what she's interested in me for, and would put me off.