Anal Play/UTI questions

So my boyfriend and I sometimes engage in anal play e.g. he fingers my butt hole or licks it or rubs his dick against it. But we are always VERY clean and very safe. For example, if he uses one finger to play with my butt, he makes sure to use a different hand entirely for my vagina so it doesn't come into contact with my urethra. But for some reason, every time we attempt anal play, I always get a UTI - it's been three times now. Why does this happen if we've been so safe?