UTI every time we have intercourse!

I've been wrought with UTIs since I started having sex when I was 16. However, it wasn't until a year ago that I realized it happens after every single time (nearly). My bf and I have been together for a year and a half, and it wasn't until I was with someone consistently could I make the correlation. It usually happens a day or two after we have sex, sometimes only 6 hours later, sometime by Monday morning (I only see my bf on Fridays because of our schedules). 
Does anybody else have this?? I've been to gynos and urologists and no one seems to have an answer...