My mom told me that I'm not allowed to have sex in her house but if I have nowhere else to sleep with my girlfriend, I'm going to do it there.
I'm wondering why you're ignoring your mom's personal request?
We all have our own places but if for some reason I can't do it at mine, or my girlfriends place, my mom's is the next best thing.
I understand that if there were nowhere else you would still want to sleep with your girlfriend but your mom has made it pretty clear that you are not allowed to have sex in her house. You should respect her rules because you don't live there and you should listen to the wishes of your parents. Otherwise, she might just bad you from coming over with your girlfriend and will likely also be pissed at her.
I'm not going to respect my mom's wishes because she doesn't have a right to say what I do in other peoples houses.
It is her house, and she has the complete right to tell you not to have sex there. If she catches you doing it then she may ban you from her house and then you won't have anywhere to go.