I have many times from several of my past partners. My ex-wife and I discussed this in depth once.
For me I wanted to suck her regular lovers cock the first time I saw it spring out fully erect from his boxers when she removed them.
I knew right away after seeing him naked and his cock erect he was physically more fit, bigger, stronger and viril than I was; we had gotten lucky and successfully found our willing alpha male she was attracted too.
Also, his cock was so much longer and bigger than mine and his ample balls, also made me feel inferior to him sexually and I knew he was going to fuck her better than me and do things I was not capable of.
She had complained I was smaller than her ex-husband anyways, so I knew it was in the her mind already she wished my cock was bigger.
Finally, After he fucked the hell out of her the first time and she told him to cum inside her pussy I knew, eventually as we got more familiar I would want and need to show them both a level of submission and acceptance of
1. Her realization and later verbal admission he pleased her way more than I ever could and she wanted to slow down sex with me for a bit and concentrate on fucking him more often.
2. His new role of her primary sexual provider and show him how thankful I was that he pleased my wife so well and how much bigger and better his large cock fucked my wife's pussy than mine.
So, I began to lick his sperm from her pussy and got to like doing it.
My ex-wife said it made her loose sexual attraction for me by doing that but on the other side of the coin it increased her desire to have sex with her lover, Eric even more.
And finally progressing to sucking his cock infront of her after he became her boyfriend and a part of our lives firmly established the line that I submitted to his superior cock fucking my wifes pussy better than me and that I also understood he was the boss when around, alpha , me beta and help carve a physical sexual role for me in their own sexual relationship as a helper and cleaner. By doing those things and more I could be present with them more and we could still be a couple even with him over at the house with us or while fucking.
Even when we would go to stay at his house I became famliar with all his toiletries, bringing things over he didn't have and keeping a pillow there to sleep on his couch as they both insisted when over there, they space from me and slept together, alone with the door shut, texting me when they needed me to bring them something like water, lube, food or a wet rag to wipe down her pussy/his cock so they could stay in bed and cuddle together.
So sensual walking in his bedroom, typical bachelor with them naked sweaty under the sheets cuddled together holding each other telling each other how much they loved and cared for each other while I wiped them off and let them know how happy I was for them and us to have Eric as a part of our lives/family and how happy he makes my wife.
When we would be alone, if she came home I would lick her if it was dried from the night before or a lunch/dinner visit to his apartment on the way home and that also, would spark deep talks between us about the state of things, her feelings about him,me and what we were doing.
So it also served as a catalyst for conversation to further solidify our maritial bond as an opportunity to tell each other how much we loved each other, were not threated or jealous by her/my feelings and a reminder its ok to be truthful with me about him when im licking his sperm from her pussy that may have been in there a day or a few hours. Kinda an "oh yea,he is licking another mans sperm from me and liking it, why should I feel bad about being open and honest with my feelings for him and us when doing that turns me off from him sexually. Often us talking how its improved our marriage, sexually. and increased her love for me because she could see I wanted her to be happy and satisfied and that my libido was low, I didn't fuck her enough or eat her pussy much and she needed and deserved to have an alpha male to fuck her and fulfill her sexual needs in our marriage and also just have fun and be with some other male who would just come over that was attractive so she could suck his cock, get fucked, dominate us both and then leave., on a superfiscial level below stepping in for me to fulfill my martial sexual responsibilities as a husband.
Which I did and also made her enjoy me doing it when he would be over and see it as she could see I knew she was in love with his superior black cock.