I can handle being the weird dude that takes super loud shits in public bathrooms. That'll just make people leave me alone.
I can't handle making my girlfriend feel like shit every time we're intimate. That would be debilitating to our relationship.
I don't mind a bit of construction work. Wouldn't be hard to tear down the drywall in the bathroom and pack the stud walls with Rockwool soundproofing insulation. And also swap the door out for a double-thickness solid core sound-deadening door with a sealing sweep. That's like a couple weekend's worth of work max.
Also she is an absolute saint and would probably be fine dealing with it to stay with me.