I take it you can't act out these fantasies with your current partner? It sounds like you're feeling a bit unfulfilled sexually - so naturally the issue is spilling over into friendships and confusing things there. Or maybe you feel this would be better experienced with women? To be honest it very much depends on the kind of s & m - as you know there are many sub-types so maybe you can now focus on finding one that would work better with your partner? Providing they approve of it of course.

There are places to go where your fantasies can be safely and appropriately fulfilled - though whether this would be ok with your partner is another matter. If they understand that you're not cheating on them, merely paying for a service they can't provide, perhaps this would be a good way of dealing with the issue.

Just a thought. Other than that i would maybe go for counselling either individually or as a couple to help stop this issue potentially overtaking other areas of your life. Perhaps there is a connection between your previous experiences with women and these pervasive fantasies - or it might just be a desire you haven't quite worked out how to deal with.

Either way the best thing would be to continue talking about it - don't quash it down and just expect it all to go away.

I hope this is vaguely useful - hopefully other members will have more advice for you. Best of luck.