potential boyfriend trouble... help!!!

My boyfriend and I are very different religions (have been dating for nearly 3 years now) and we have been seriously talking about marriage and stuff like that for a long while now. He doesn't see the religion thing as an issue but it's a huge issue for me but we've never really talked about it... I'm just so scared if I give him an ultimatum he'll leave me or something. He is the love of my life but I'm not going to put myself through an opposing religions marriage and eventually put our future kids through that because I've seen plenty of families that get torn apart by different religions of the parents... help