Well DD, I'm sure you already know this but, many men find the idea of sex with another man revolting but they find sex between 2 women a big turn-on. So to fantasize being one of those two women is not all that far off from that.
For many of the men that I know, women are basicly objects of sexual desire first and human beings second, and since their primary motivation in life is sexual in nature, it might be hard for them to understand that many women are NOT attracted to other women. Hell, most men haven't a clue about how and what women think, and a lot of them don't much care either, as long as they get what they want.

There's nothing wrong with having a fantasy, I have lots of them, but just expecting someone else to partake in that fantasy without even caring if the other person is interested and willing, there's where the trouble starts.