How to deal with my mom finding out that I'm ually active?

So, here's the deal. I'm 20 years old and attending college in the same city that my parents live in. I live in a house downtown with a couple of my fraternity brothers. I'm currently in a relationship and I am ually active. My mom and my sister came by the house unannouced and when my sister opened my nightstand drawer she saw an empty condom box and of course she flipped out(she's 23 by the way). She didn't tell my mom right then but my mom called and told me I needed to come home to talk about my "behavior." She yelled at me for a good two hours and told me that I would have to move back home or she would cut me off. I understand that she is upset but I think she is being completely irrational as I haven't done anything that bad to warrant being forced to come back home. Any advice to get through this? I really need it.