Have you ever witnessed someone else feeling up your wife/girlfriend

In a sex club, I would say it is a normal phenomenon.
Because we visited such a club once a month, my answer can only be YES. I witnessed it countless times.

I cannot remember ever seeing it in public. My wife did tell me about it later, though. It happened several times in the naturist village of Cap d'Agde.
Incidentally, it also makes a big difference whether it is skin on skin or through clothes.
We explored our hometown with her regular black lover. In a rather busy pub, I witnessed him caressing her buttocks.

At home. Yes, many times, and with me as a witness.
It was incredibly arousing to see my wife open her legs for him (that black lover).
Attached is a photo of the very first time he visited us.

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I took my wife to a restaurant to hook up with a guy. She wore a skirt with no panties. We sat in a booth and as we drank, she put his hand on her thigh and ran it up so he could feel her wet pussy. He began fingering her at the table. She bit her lip as she began cumming. He licked his fingers saying we should go for a ride. As I drove he was in the back seat fucking my wife. He finally came and we dropped him off and I took her home and banged her cum filled pussy. Adding my load to his. Her neck had hickies all over it. We were in our mid 40's having hot fun.