Wow! What has been said/suggested makes a lot of since. When women are excited, they get wet right? Some women produce more wetness/fluids than others when they are excited/stimulated. Those fluids in the heat of passion, runs down from the inserted penis, down the penis shaft, and between your/the man's legs and possibly/most likely to his anus!
Give or take some persperation mixed in, you're both wet.
Kind of put another spend on things when you stop to really think about it huh.
So if she has an infection/std/yeast, she passes it on to her partner