Sandi, no one here is going to isolate you for affairs. This forum is for reinforcing MWD's principles and to support visitors through that program.
your point about fantasizing interfering with a loving relationship wtih your spouse is spot on. Successful couples share their fantasies with their spouse, they don't shut them out of them.
If you are fantasizing about firemen, and you share that with your spouse, maybe he can dress up like a fireman for example, I dunno.
But shutting your spouse OUT of your thoughts and dreams is a road to marital oblivion. Open up, share, trust, forgive, and love your spouse so they CAN love you back. No one can effectively love a brick wall.