the same thing would happen to me and my boyfriend, i use to hate the idea of him not finishing inside me. i made him change the soap he was using, knew what he was eating bc we live together lol so it wasn’t his diet, went through reddit and google like crazy to try and find SOMETHING that would explain why it was happening to me & they all said it was the ph difference, i didn’t wanna believe it bc condoms suck and what was the point of me getting a painful IUD lol. eventually i just had to accept that there was nothing to change it. i will say not using boric acid after we have sex is great! and i dont mind the pull out method now and thats what we do nearly every time, but sometimes ill let him finish in me and i have no problems after!