When I have my period, I get very horny and want my BF to have sex with me. He doesnt want to, he thinks it's gross. Why am I so horny then.

It ain't you, Baby, its him. A woman is just more sensitive during her period. Intercourse actually eases the cramping and can make you feel a lot better during those uncomfortable days of the month. Tell him it would help you with the cramps and make you a lot easier to be around. If he still doesn't want to, keep it in mind as your time together progresses. His immature attitude might get a little old fast.