Mensturation itself doesn't seem to be so gross, but after seing some videos and listening and reading some talk, in biology lesson, it gives me discust. But well, human creatures are discusting anyways.
Just imagine your food digesting, body liquids are moving and nice slice of pork is turning into slippery goo and its sliding down towards the anal gap as its producing gases while turning brown. And those gases are reaching into someones slimy nose, you could actually feel the tiny molecular shit parts touching your little skin fringes between those tiny capilars, you could actually sense some smell what marks a reaction of the described situation. Have you ever thought about what all is behind those simple actions we do every day? You are breathing someones urine every day, so yeah, from this point, mensturation should be normal thing as we are uncountiously used to such things.