Loneliness in the Dark

I'm sitting here in our bed, after a long day of not really doing anything for grad school. My partner is sleeping next to me, but I feel so far away from him. We've been together since before either of us started these programs and once, we used to have this connection that was unbreakable. But lately, it's just... nothing. Tonight, after he fell asleep, I got turned on out of the blue, for someone else. An old fantasy came back, involving a stranger with dark hair who would take me forcefully in an alley somewhere. The thought is driving me crazy and I'm hard as hell. But instead of touching myself, I just lie here, feeling dirty, guilty, and lost. Is it normal to have these thoughts? What do I do when they're all-consuming like this?