Cuckold instincts start first when a man begins to realize that he's sexually inadequate. First he starts to wonder how much better he could fuck his woman if he was bigger, or better. Then he starts getting addicted to the pleasure a woman could have with a real cock. Yes, he wishes he could give that to you, but he knows he can't. Yes, it hurts, but soon his dick gets hard when he thinks about you getting what he's not man enough to give you. It'll never stop hurting and he'll never stop wishing he was man enough to give it to you himself, but he never will be and he knows that and the cuck instincts get stronger and stronger with your every moan and every orgasm... Until he's turned on completely by your cheating sex, by you fucking better cocks, by other men cumming in you, and some men even grow to be turned on by other men getting their wife pregnant. It's an instinct that gets turned on that eventually the man can't turn off again. The need for his woman to have amazing sex, ending in the best possible seed in her womb. That's hardwired into our dna.