Recently, w/in an hour after sex w/o showering, i break out in a rash/hives in my groin/upper thigh/buttocks area. no blisters, just gradually go away?

A couple: .. possibilities come to mind when considering hives that consistently occur after intercourse. Cholinergic urticaria (hives triggered by heat), contact sensitivity (latex condoms? although this may more commonly be associated with poison ivy-like rash), seminal plasma hypersensitivity (semen allergy-usually person specific, but also usually a burning sensation). Can try otc cetirizine.