Leviticus does say that touching a menstruating woman is unclean, but Christ Himself touched a hemorrhaging woman in the Gospel. There is no additional restriction on marital intimacy when a woman is on her period besides what the couple is comfortable with.
My fiance and I have talked about this as well. It’s just one of those things we’re going to have to see. Every woman’s cycle is different, but at least for me, when it’s heaviest, sex would probably be the farthest thing from my mind. It would just be too uncomfortable, with cramps and everything, and probably quite messy. But for lighter days, if the mood strikes and we’re both comfortable with it…we’ll see.
Also remember with NFP that menstruation does not necessarily mean infertility. I remember seeing a chart of the different phases which was showing how the length/symptoms differed slightly in various women, and phase 2 can sometimes start before menstruation has finished. If I’m wrong on this, someone please correct me–but I think you’re supposed to keep cross-checking for other fertility signs even while menstruating.