Well, I imagine there are as many different answers as there are types of cucks. I'm not small and am actually larger (and thicker) than most men. Truthfully, I've always been able to sexually satisfy woman and I was usually the one to cheat on them.
But, there is something absolutely exhilarating to being totally submissive and, in the process, humiliated. I don't know why I feel that way (I'm sure somewhere it's connected to my childhood as everything is) but I know that I love it.
The first time I was ever with a guy was in high school. I was a senior, he was a friend. I had made a lot of sexual references to him and, finally, one night, when we were both in a drunken stupor, I finally got to blow him. It was a night that stays embedded in my mind for a lot of reasons. He took his pants and underwear down (we were in his car and we had parked to drink some more). When I bent over to suck him, he seemed pleasantly surprised that I was really doing it. After a minute, he asked if he could come in my mouth. My initial reaction was to stop and say no way, but, really, I wanted him to. So, I figured, why not do what I wanted to do. I stopped again and said "OK." Now, he knew what I was saying but he said, "OK what?" Really, to have to say it got me that much harder, so I said, "You can come in my mouth." He did and when he was finished, he asked me how he tasted. I mean, we had joked that this is the way we talked to and treated women and here I was being the one humiliated. I told him delicious. As that summer progressed, I blew him many times, each time seemed more humiliating than the last and each time I liked it more. Once, I'll never forget, he had just gotten off work. He had worked for a township road crew and had been sweating and was filthy. He called me to his house and said, "Do you want to blow me before or after I take a shower?" I dropped to my knees and did it then. Just amazing.
The first time I opened up to a girl was in college. I was dating a girl that I can honestly say got ***** and fucked anything that moved. Once, I went to her apartment and caught her in bed with another guy. The guy left and during our long talk, I started massaging her feet. She knew that was my sign for I'm ready to fuck you. She asked me what I was doing as I had to know that another guy had just come in her. I told her I didn't care and, in fact, it turned me on.
Fast forward a couple of weeks. That girl and I were out and who do we see at the bar but the guy she had fucked. She walked over and gave the guy a kiss and told him to join us at the table we were at. She sat across the booth from me and pulled her socked feet out of her shoes and had me massage them. The guy sat down and she talked about their night as if I wasn't there. He seemed uncomfortable but she finally pointed at me and said, "Don't worry about him, he liked that we had sex." The guy snickered at me and I got hotter than I already was. The upshot, I drove them to her apartment (they made out in the back seat while I drove) and dropped them off to fuck. He refused to allow me to come in, but, said that I could come by when they were finished. I dated her for a few more months and the whole time, he openly fucked her. Why this humiliation turned (and turns) me on, I don't know. But, even with that relationship, the guy got more domineering and I got more submissive (to the point that when he would be at her place fucking her, he'd call me and have me but them beer and cigarettes if they ran out). The cap to it all, he once had me smell and lick the insides of his filthy tennis shoes while they fucked. Worse, every few minutes before they actually fucked, he'd come out the bedroom and make sure I was doing it. On one occasion, I had stopped and he was so angry that he took his underwear and stuffed them in my mouth. When he came back 10 minutes later, he removed them and told me he'd better not catch me fucking up again. He didn't. I smelled and licked those grimy tennis shoes for hours. The next morning, I had to go to a department store and buy him new shoes and underwear (he refused to wear what I had in my mouth).
Even as I write this, I've become totally horny. So why do I like the humiliation, I don't know, but I absolutely crave it. The trick is being able to get past your natural defenses and submit to it. Once you do, it's wonderful.
EDIT: I forgot to put in my original thought. It is a combination, IMO, of the need to be submissive which is connected to the desire to punish myself for all the things that I think I did wrong. You know, it's not one girl that I fucked over (or even one guy, that high school chum who I loved blowing, I simply blew off [pardon the pun] when I'd had my fill of him - he never knew why I stopped calling him, talking to him, etc.). I screwed over so many sexual partners....oh well.