OK, you want a serious answer, here it is. Not every man's penis goes soft when he is being anally penetrated. Some men's do, but others' become even harder and many men can reach an orgasmic climax and ejaculate while being penetrated. A few men can reach a climax solely from the prostate stimulation of the penetration without any stimulation to the penis.

One has to look no further than gay porn to confirm this. It is not at all unusual to see a man fully erect and ejaculating while he is being penetrated by another man.