My boyfriend enjoys anal sex frequently. now he has a itchy rash on his shaft. i have no symptoms. could it be from anal sex?

Yes: Any rash on the skin of the penis should be examined by a doctor. This could be anything from a yeast infection to a bacterial infection.

Multiple causes: Rashes on the penis can be from a number of causes including an irritant contact dermatitis or fungal or yeast infection. An evaluation by a physician and possible culture will help clarify the cause of the rash.