I must start by complimenting you for not rejecting the idea right off the bat. That shows some real strength of character and a non-judgemental attitude. Props for that.

Now that you've had time to think about it, can you narrow down the specific part of the fantasy that pushes it over the edge for you? Is it the whole scene, or some specific aspect of it? I think the best thing you can do is to try to figure out exactly what about the fantasy you are uncomfortable with. Once you do that, you'll be able to talk about it with him more objectively.

I think honesty is the best policy. Maybe tell him that this fantasy is a little over the top for you. Maybe tell him that this fantasy is WAY over the top, if that is the case. The fact that you aren't judging him for it is a big plus. Just tell him that this fantasy doesn't work for you, and ask him for another.

In my opinion, the key to any relationship is communication. He was open with you, and trusted you enough to share his fantasy. That trust appears to be well founded because you didn't freak out when he told you. You just wanted to think it over. Just be honest with him, and trust him to accept and respect your feelings about it.

Well, what was the fantasy? Maybe there is a way you can compromise. A way to fulfill his fantasy in a manner that you're comfortable with. It's always good to be open minded about new experiences.

Without knowing what the fantasy is, like bdvnt said, break it down into components you can work with. Give it your own spin in a direction that makes you more comfortable. Fantasy play is just that, fantasy, as long as the lines between what you are playing and what you are doing don't blur (especially if it's something you would not act out in real life) find your own spin on it and have fun.
If it's too over the top for you to even be comfortable playing with, then tell him so. Don't do something that makes you uncomfortable because in the long run that wouldn't be fun or sexy for either of you.

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.

Thank you very much, I will talk to him about it. I guess my issue with it stems from the fact it triggers some issues from my past. I will talk to him and hopefully we can work it out.

Glad to report that my husband understood and we were able to make an acceptable compromise I love my hubby