Is it always an UTI? Could you be allergic to condoms? If this is in fact an infection it could be a number of things. Like laundry soap/body wash or some lubricants. He could be using lubricants by himself and not realizing they affect you. I couldn't rule out cheating but it can't be ruled in either. Good luck I really hope it's an easy quick solution:)
possible but it may just be your vaginal tract is just not lubricated enough. try unscented lube. hope this helps
Do you urinate after sex? That helps prevent UTI's!