Re: itchy rash on my penis
Does your girlfriend ever have any symptoms of anything? Does it ever burn or tingle to urinate? It could be a symptom of a more serious infection especially if there are any other symptoms.
Could it be a reaction to something you use as lube? It might just be an odd allergy or irritation from friction. But chances are pretty good it's just a yeast infection. Yeast lives on our skin and sometimes too much grows in places like in our ears, between toes, or on/in the penis and vagina. If your girlfriend is not having any symptoms of yeast infection (like vaginal itching?), then it must be very mild if she has anything at all. It could just be that her natural vaginal compounds on your skin make you susceptible to yeast infection.
I would recommend washing your penis off within 15 min after sex. Don't scrub it hard or anything (that will cause its own irritation), but lightly rub all the skin oil and lube off with you fingertips. You might find that an anti-fungal soap helps (if it's not too irritating). I would probably at least try a gentle soap that rinses clean like baby soap. If cleaning better and sooner after sex prevents it, I would think it was just yeast.
Drinking milk can make yeast problems more likely. Good eating and healthy digestive bacteria help prevent yeast problems.