Not every guy that's likes to watch his wife fuck is a cuck. Some men just take great pleasure in their wife getting fucked by another man because, he is excited by her pleasure and sexuality. He doesn't want to be ridiculed and he will probably kick your ass out the door if you do so. You are to respect his wife, your only duty is to make her cum while he watches. Your job is to make sure he has the best view of everything.

If she's sucking your dick, make sure he has a good view and she is in a position to look him in the eyes especially, if she wants and let’s you cum in her mouth. If you are fucking her doggie style, make sure he has the best view of your cock sliding in and out of his wife, as a matter of fact that goes for any position you may fuck her in. If he wants to eat her pussy while you are fucking her, don't freak out, he just loves the view. If he stays there when you cum in her, it's because he wants to taste you on/in her, because another man just fucked her and it excites him tremendously, it does not mean he's gay and wants to suck your dick. But, if he does what is the downside. After all, you just fucked his wife whom he treasures. Count yourself lucky and then maybe, just maybe, you'll be invited back to fuck her again.