Yeah, that's a toughie. I'm a bi dude in a gay relationship like you, and I was in a similar position a while back, bc dorms were off limits to visitors, his house was off limits to gays, and his tiny car wouldn't fit us considering we're both over 6'. Luckily, my roommates ended up being cool with me sneaking him in, but for a while there I was considering other options. The best I think I can offer is finding a secluded area, somewhere nobody goes at night (and make sure nobody goes there :P), and trying it there. Not very romantic, I know, but if you're desperate, anywhere can work in a pinch. Or, if you have like, a super-hella-chill friend, ask to do it in their place lmao