If you find someone hotter than someone else, it is just easier to get worked up for all kinds of action. That is a pretty stark reality.I think this is a great example of the different perspectives not understanding what the other is actually saying. As insightful as this is, this is not really an explanation for the actual "you did it for him, but not for me" situation.
A closer analog would be if Tom had said he wouldn't do it for you because he hated oral, or that he wasn't really into oral, and didn't like it with his ex wife, or you just picked up on him not being really into it, and so you take it off the table because you were understanding and didn't want him to be doing something he didn't really enjoy specifically and exclusively for your pleasure. And then some time later, you find out that not only did he love it with his ex, and did it all the time, and actively and enthusiastically pursued oral on her, and by all accounts he was amazing at it, her vagina was better than unicorns and puppies.
Perhaps a better way to frame the question, more accurate to what is actually being asked is "what do their other lovers have that I don't?"
It is also maybe more prevalent among the less mature?
There isn't much I wouldn't do for a partner to blow her mind but when I was younger, I experienced some situations like this.
I was involved with a wonderful woman who was taller than Uma and just as hot with bigger, nicer, breasts. I actually really cared about her. Due to immaturity, I left her for a belly dancer who was crazy hot.
Sex with the Amazon was wonderful and nicely erotic but I never went down on her.
Sex with the belly dancer was off the charts hot and I did things with her that I never did with anyone before including oral.
I didn't perform oral sex again for anyone until I met Mrs. Conan and she had to ask and teach me how to perfect it.
I'm a lot more grown up now and would do anything I have done in the past with a current lover.
I honestly don't personally understand doing differently unless there is some health issue associated with a certain sex act.