Sore Penis If Had Sex Multiple Times, After Ejaculation. Reason?

Hi, I am a 42 year old man in excellent health. If I have sex multiple times in one night I find that my penis becomes sore. I do not mean the skin or outer part of the penis from simple friction. I mean the interior of the penis becomes sore -- mostly the urethra . It seems to be a result of ejaculation . I say that because whether I ejaculate after a short period of time or make it last to marathon lengths, it is the same result. Even when it becomes sore after two or three times, it still becomes erect with no stimulation from her or me, so it is a bit confusing. I am wondering if it is a) a problem with the urethra (I have never been able to urinate accurately), or b) I am not hard enough (theory being that increased hardness would protect the penis from the beating it must take during intravaginal thrusting). The problem with (b) is that this problem seems to be the result of ejaculation. Another fact worth noting is that this problem seems like it may be more pronounced with women who have more narrow vaginas. Thank you for any suggestions and help you can offer.