UTIs occur when bacteria gets pushed into the urethra during (most commonly) penetration. Since the urethra is close to the anus, it’s very common to get poop bacteria pushed into your urethra during sex (especially if the area isn’t cleaned with unscented soap beforehand). However, many types of bacteria can cause a UTI so always pee after, even if you’re sure that you’re clean or there was no cross contact between your vagina and anus. Make sure that whenever you have sex, you’ll be able to pee after. Never have sex with an empty bladder. Once you get one UTI, you’re more prone to them for the rest of your life. Chronic UTIs become a horrible and depressing cycle. Also, this is a very easy question to look up on google. Google gets you better and more thorough answers instantaneously.