The only time this happened to me was when I was DP my wife with another guy and I was on the bottom and his dripped down on me after he came in my wife's ass, I had her tits in my face and was about to cum myself so I didn't mind at that moment!
When I have sex with my wife I can usually hold off cumming until she has gone a few times when we are in missionary. But when she turns around into doggy style position I’m on borrowed time! I just love looking her ass!