Always use condoms when dating someone new. Condoms are the only proven method of preventing sexually transmitted infections (STIs), according to Gokhan Anil, M.D., an ob-gyn with the Mayo Clinic Health System. A 2013 study published in the journal PLoS One revealed something you might not be aware of, though: They can also assist in maintaining your vagina's pH level consistently throughout sex, ensuring that all the beneficial bacteria can continue to thrive there.