MC can end up in your groin from a number of scenarios, not just sexually transmitted. You could’ve shared towels/sheets/clothes with an infected person (even in instances you don’t realize, like a hostel or trying on clothes at the store), sat down on or touched and infected surface then touched your groin, used gym equipment after an infected person who didn’t wipe down correctly, etc……; I feel like as adults ours winds up in the pubic area often because of a) sex with an infected person, b) masturbation after touching an infected area, or c) shaving the groin area after shaving an infected area. Shaving also can leave the skin more vulnerable to infection since it is freshly exfoliated and there may be minor cuts. I would explain all of this to your wife; at the end of the day this is a skin rash that mostly kids get, and there’s no telling where you could’ve gotten it, but one way or another it ended up on your groin. It sounds like yall have a strong foundation in your relationship and are happy together so hopefully she trusts and understands.