You always end up at least admiring the cock that ends up in your wife. It's a natural progression from giving your wife's pussy to the guy. Now it's HIS pussy to fuck and you have to know that it must be a great cock or it wouldn't be in there. Your wife likes it in there and if it's good enough for her then it's good enough for you. You end up thinking that you should experience at least some of what your wife is experiencing, so you start to fantasize about sucking it the same as your wife has done so many times. You already swallow his cum when it leaks out of your wife, so why not suck it and get it directly?
Then you talk it over with your wife, and she agrees that it would be fine with her as long as you don't get addicted to sucking cocks. But you only want to suck this one anyway, so arrangements are made.
The day comes and your wife and her lover sit on the couch, completely naked. You kneel before the guy and begin to suck his dick. Your wife is kissing the guy on the mouth and jacking him at the base of his cock while your head bobs up and down on the penis that has penetrated your wife so many times. After about 15 minutes he moans and starts to cum, releasing big streams of jizz into your mouth. It tastes very familiar to you and you swallow. You clean up any that dripped down the side of his cock.
Your wife smiles at you. The guy gets up and heads for the shower. Your wife spreads her legs now. You have to eat her now because she's turned on by the fact that her lover just squirted his cum into her husband's mouth. You lick her clit and she cums quickly. Moments later she gets up to join her lover in the shower. You watch her hot body walk away. You feel completely humiliated, but happy. You stroke your cock and cum in about 5 seconds.
Later, you all go together to get some dinner like nothing's happened. But you play it all over and over in your mind and you agree with your wife that the guy's dick is the greatest thing in the world.
After some time, you end up being the lover's regular cocksucker while your wife is on her period.
Been there. Done that.