My girlfriend has bacterial vaginosis that has not been treated. Every time we have sex, I develop a uti that last about one day. Is it due to the BV?

Test & Treat. Studies show that when one STD is present, others may be as well. You and your partner should abstain, get full STI testing, and get treated and abstain until treatment is completed. Full STD panel includes Gonorrhea, Chlamydia, Syphilis, Hepatitis B, (+C if drugs/male/male sex/risky encounters) and HIV. Stay calm and get tested and treated. Advise all partners to get tested/treated.
Here are some... It is hard to say if you got UTI, which usually does not last only one day. To solve the mystery of your symptoms, it is logical to follow the suggestions described in https://ebettercare.com/feel-become-sick/ and in articles on UTI on http://formefirst.com/UTI.html & http://formefirst.com/toptenmythsUTI.html. Thereby you gain insights on how to work well with Dr so to reach right diagnosis & Rx..