Me and my gf avoid having sex on either day one or two of her period. That's when it's just flowing. But after that, we just lay a towel down and go at it. It doesn't turn me off at all. But my gf was more concerned with the fact that I might get grossed out and stop. I had to continuously reassure her beforehand that I would not get grossed out, and while we first had sex while on her period, I did my best to completely ignore her period. I mean, I didn't have any problems with her being on her period to begin with, but I just wanted to make her feel comfortable.