My fiancé of a year and a half cheated on me with her gay best friend. I am still forgiving her and feel like I am making progress. But whenever I bring up sex or anything sexual she can't stop thinking about him and she breaks down. How can I help her move on. What can I do. She keeps pushing me to break up with her but she means a lot to me and I am not willing to just give up.

So she's in love with her gay friend... there is nothing for you to 'do' but wait patiently to see if, when, how she gets over it on her own. Is he still her best friend or did it get awkward after the sex? How long has it been?
I'd say that the less you dwell on it the better. This isn't something you help her with. But if she persists in telling you to break up with her, I would plan a little vacation without her, maybe go see family or go somewhere with a male friend. I predict she will miss you.

She obviously has some feelings for this person or is questioning her feelings for you. You can help her move on by letting her go like she has asked you to

This is directly out of The Birdcage (http://www.imdb.com/title/tt0115685/).

there is nothing for you to 'do' but wait patiently to see if, when, how she gets over it on her own.
This is awful advice, equivalent to sweeping the mess under the rug instead of throwing it in the garbage can where it belongs. Yes, there is something else for you to do: leave her. Do not wait patiently as this will prolong your torment and she will definitely cheat on you again. The waiting game will not fix this.

If she can't resist sex with a gay man, then she definitely cannot handle herself with straight men.

And this I don't understand: she wants out of the relationship but won't end it herself and so she is forcing you to do it for her? How senseless is that? This is a testament to her weak and disillusioned state, she's not thinking clearly, hence, she is in no shape to be in a relationship.

Break up with her and don't look back. Thank your lucky stars that you did not marry this woman.

She wants out, he is the one not wanting to let her out.
Yes she cheated, he forgave her and she still wants out. I think maybe guilt keeps her there, but he needs to let go.