I know this is probably gonna sound harsh, so apologies in advance. I don't know any other way to say it than straight up.
When you speak about "resolving this issue" I get the distinct impression that what you really mean is "him stopping this behavior." That's not a resolution or a compromise, that's him changing his behavior to meet your demands. You cannot change him. You don't have that kind of power.
So, either you want to be with him--as he is--online sexual fantasies included, or you don't.
It sounds to me that this kind of behavior is unacceptable to you. Nothing wrong with that, you have every right to define what is acceptable to you and what isn't. What you don't have the right to do is change another person. You can change your mind about what is acceptable, or you can find someone whose behavior is acceptable. You can't have both.
L