Best humiliating line your wife came up with.

my wife was sucking off a neighbor 1-2 times a week. she always complained that she really didn't like him. i asked her why she continued sucking him.

her words; "he has a big dick & i like sucking it".

works for me.
We had a similar situation with our neighbor, but minus the cock sucking, as she couldn't fit his cock in her mouth. With her tiny Clara-Bow mouth and his double-wide penis, which was not round in profile but a flattened oval, like a cobra's head, the geometry wasn't in their favor. (His cock looked something like a double-barrel shotgun, as it looked like two normal penises side-by-side, wrapped in skin.) He did, however, fuck her as often as we could mange it, sometimes as much as three times a week. (He had a live-in girlfriend who was entirely unstable, partly due, no doubt, to his constant cheating on her. If he had lived solo, I cannot imagine how often he would have been in between her legs.)

The thing was neither of us liked him, as we deemed him untrustworthy and scatter-brained—and not too bright. He could not be taught how to lick pussy, for example. So why did she repeatedly seek his big cock? She like the stretched feeling afterwards. In fact, she called being fucked by him pussy stretching time. My God, did she feel stretched after he fucked her. Fortunately, she returned to her natural tightness in a day or two.

When we desired his presence, I placed an artificial rose stem through our back fence's staves, which he could see from his kitchen window. Often in was over in a flash; other times, not at all.

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Clara Bow
 
My wife would never aim to or want to humiliate me. We don't have that kind of relationship. Which makes my humiliation all the more real when she does it inadvertently.

Some the most humiliating examples...

She had spent the night at a hotel with her largest hung bull. He's 8.5 inches and fat. He worked her cunt thoroughly and filled it several times with a big load. When she came home I was keen to fuck her while she talked me through it. It really turns me on. I got her on hands and knees and enjoyed fingering her cunt open to watch his cum flow out. I soon started to fuck her. After 5 mins of sloppy fucking she said she can't wait and to please fuck her now....

Another occasion, I was in missionary the morning we were expecting a young bull to come by and take out for the night. He messaged her mid fuck to say he wanted to drop by earlier and go to their hotel where he'd arranged an early check in... Obviously wanting to start their fucking in the afternoon. My wife, conscious to be ready for him, asked me to hurry up and to please withdraw.... She clearly did not want my cum in her vagina when he went down on her.