Reoccurring uti

Why does sexual activity trigger flare up with uti? I go toilet after doing anything intimate and still get a uti. I’ve had uti’s on and off for about 18 months now. Never doing anything sexual while I have symptoms or on strong antibiotics. I am on a low does of antibiotics and have been for 9 months now but still get uti’s. Also take d-mannose. It’s not even from actual sex. It’s mainly from fingers for some weird reason. And yes the guy is hygienic. I’ve been tested for stds & im clean. I don’t have thrush anymore had it 5 months ago but still get uti’s. i wipe front to back, I got given a special body wash without perfume in incase that was triggering the flare ups. I get bloods done when I get infections & they say my white blood count is high which I’m assuming is down to the uti. I’m sure they’d check for more serious issues? I make the guy wash his hands before touching me and I’m genuinely concerned now as there’s nothing more I can think of to stop getting infections. I get positive uti results by urologist and they still can’t find the cause so I’m wondering how to stop because at this point I feel like I’m going to suffer with them forever and can’t live my life properly.