Aside from all the obvious reasons — protection from unwanted pregnancy, protection from any one of the myriad of STD's — there's yet another compelling argument for literally Always Using A Condom. Researchers in Australia found reason to believe that having unprotected, penetrative sex with a man who didn't wrap it up can disturb a woman's vaginal balance, also called a biome. Or, in other words, an unprotected dick can disrupt the balance of bacteria in a vagina, putting women at risk for bacterial infections like bacterial vaginosis and UTI's.
A team of researchers from the Melbourne Sexual Health Centre in Australia tracked the bacteria living in 52 women over the course of a year. The women in the group, who were all young students and who had not all yet had penetrative sex at the start of the study, were told to swab their vagina every three months and keep a record of their sex lives. This meant they had to keep a log of the type of sex they were having and whether or not they used a condom. The team of researchers then compared the sexual diaries of these women to the vaginal swabs, to track any changes in bacteria levels.
The study results suggest that, just like STD's, non-pathogenic bacteria can also be sexually transmitted via unprotected penile-vaginal sex. What does that mean for women who are potentially having this kind of sex? A healthy vaginal biome is typically dominated by one strain of bacteria. The study found that women having unprotected P-in-V sex had vaginal biomes dominated by higher levels of two strains of bacteria — which puts them at risk for urinary tract infections, bacterial vaginosis and possibly premature birth.
Janneke van de Wijgert, a professor of infection and global health from the University of Liverpool, told New Scientist that the study results don't mean penises are necessarily bad for the vagina. Van de Wijgert believes a woman's vaginal biome can get used to a long-term partner's particular bacteria strains and levels. "The risk lies in having sex with a new partner, which is a microbial assault on the vagina," she said. "The vagina will mount an immune response against the bacteria, causing inflammation."
If you've found that you tend to get a UTI after having sex with a new partner for the first time (and you aren't using a condom, which is risky anyway!!!!), this could explain why. Your vag isn't yet accustomed to this particular dude's penis bacteria. Isn't sex a beautiful expression of love! So, let's all say it together: WEAR A CONDOM.