Well, 6 years is a long time to be in a relationship with someone and assuming it is yourself you are talking about, is it worth the risk of losing the one you are with now to fulfil this fantasy? Sexual fantasies or crushes of other people while you are in a relationship with someone else is normal, at least to me. But it's up to you to decide if your 6 year relationship is worth more than what could potentially occur with this other person.
Talk to your partner about it. Communication is key. Maybe all you need to do is spice things up in the bedroom.