Both My Man and I have some pretty kinky fantasies. They are not always the same ones, however. When they mesh, it's perfect.
But, something he is thinking about doesn't do it for me. I might try it anyway (depending, there is little I say no to) but if it's something I know I won't do, I'll say something like, "That's a really imaginative fantasy. I don't really want to do that right now, but how about you think
about it, and I'll give you a blow job, or we can do something else, and it will be close? OK?" He does the same kind of thing when one of my fantasies is just too out there for him.
Then, we just move on. You really can't help the stuff that pops into your mind when you are trying to have an orgasm. So, I never mock and he doesn't either. We talk about it, and decide whether to try it or not. If we don't try it, the one who was thinking about is is free to continue to use it as a fantasy.