How to Do It Photo illustration by Slate. Photo by laflor/iStock/Getty Images Plus. Our advice columnists have heard it all over the years—so we’re diving into the How to Do It archives to share classic letters with our readers. Have a question?   Send it to Stoya   and   Rich here.  (It’s anonymous!) Dear How to Do It, I have a problem with my partner of six years. It has to do with him going down on me. Almost every time he performs oral sex on me I get a UTI. It really is terrible because he is so good at it, and it’s one of his favorite things to do. I know I’m really clean and also, I go yearly to my gynecologist and get an STD screen just in case because you never know if someone is cheating. I thought maybe it was his mouth. He went to the dentist and had several crowns and work done. Is it possible that his “flora” is just not compatible with mine? Also, he’s well endowed and takes a while to climax, so there’s lots of friction.…