I have had sex with my girlfriend on her period many times... we in fact prefer it. She is better lubricated, so it's more comfortable for her, and she's really into it when she's 'on' so I enjoy it more. I don't quite know how to describe it... but it's like there's some primal attraction, raw animal lust, the pure need to physicaly satisfy. It's intense to say the least. In terms of technicalities like messiness... well... we have a beach towel for just such occassions that we don't mind staining, and a quick shower washes away all 'sins'.