Why does my penis get red and irritated like a rash after sex i've gone to the doctor but it was not red so he said everthing was ok?

Hard to say: Due to friction? Yeast infection?Have you tried using a condom and see if redness does not occur?

I can't see it: Either, so i can't tell you any more than your doctor did. Maybe you're having sex in a way that's different than i'm used to, such as utilizing a blender or something. Only suggestion would be to discuss it with your doctor again.