My boyfriend says lately a few days after we have sex his penis feels irritated and had a rash?

Post sex irritation: Without additional information this is only a guess. Often times penile irritation following intercourse can be due to either vaginal infections or changes in vaginal environment such as pH changes. Various things can cause a change in the vaginal environment including progesterone only birth control which can cause in some women a low estrogen state in the vagina.