If the two of them lined up in front of you now with eyes closed mouths puckered arms out which one would you step to?
Thats your big answer.

Are you scared you'll lose her?
I know two couples who are married and one of the sides has done the same thing and come clean straight up on both occasions,both were actually the females in each case and both couples are now stronger than ever with the honesty commming to the front straight away helped the other one see it for what it was, A slip that never turned into a slide cause their partner (you in this case) thought better of it and pulled the plug before going the whole way.

I've been married for 16 years never kissed another woman but mate Ive been close so close in fact I may as well of. Its no big one.We are all human.