I believe I read this theory on some other site. I think it was called sperm competition. When a man sees or thinks his wife/girlfriend is with another man when they have sex with them they tend to cum a lot faster to make sure their seed is deposited quicker to impregnate before the bull does. Longer cocks will indeed remove a lot of the sperm through thrusting. So cuckolding can lead to premature ejaculatation for the cuck when a bull is around or when the cuck thinks the bull will be fucking his wife/girlfriend soonIt's simply a biological response.
He is trying on compete with a more well-equipped penis. In order to do so his body realizes he needs to shoot first in order to have a chance a fertilization.
The larger head of the black dick will "scoop" out any sperm. Any naturally due to the longer length it's able to do this at deeper depths. So your husband body is trying to get some good swimmers a much needed head start!