First of all - he's not a freak.

Fetishes aren't rational - people don't choose to have them. What you haven't made clear is whether this is an issue threatening your relationship or something he isn't really that concerned about you participating in. All you've said is that you're not comfortable with it. There have been a lot of couples who can't perform something the other partner wants and have managed to remain a couple. I don't know how serious this situation is but otherwise you can only leave him and work on your marriage - or leave both men and improve on your own happiness. Clearly this man isn't happy in his own relationship to go ahead and cheat on his unsuspecting girlfriend, I wonder why you'd want to add his unhappiness to yours? Perhaps a clean break is advisable, surely continuing with something you're unsure of will only lead to more hurt in the end?

Definitely discuss the issue, I don't think you can expect this person to change - they know what it is they want, but I think it's important to find out just how much they want it. I mean if this person has already cheated it wouldn't surprise me if they did start looking elsewhere for someone to better meet their needs. It sounds to me like they have particular difficulty in expressing just what they want.

At least you're in a position now where you can choose what it is you want. Good luck.