You see a woman's body calibrates to the intersection of her vaginal flora with his semen & flora. Her body knows his body and her flora and immune system get accustomed to his flora, pH, etc. So you don't get weird discharges, infections, smells etc in a monogamous sexual relationship, under normal circumstances.
So if your friend decides to dip his djck in strange vagina? Guess what happens? The new woman's vaginal juice is dfferent that the monogamous woman's, and voila! You get bacterial vaginosis (BV) which is foul smelling bacterial overgrowth in the loyal woman's vagina from the introduction of another woman's biology/flora, carried home in the urethra of the man (your penis scoops up microbes from your sexual encouters, especially if you are raw without a condom), and this woman to woman cross contamination from the man's promiscuity causes the BV infection in the monogamous female partner.
Not cool. Same can occur with yeast or candida.