Am I just over reacting or am I legit to be mad?

So bit of back story my partner has been friends with the God father of our daughter since high-school, he has been In a marriage for years and for a time was open and then his jealous wife closed it ( he was sleeping with other girls thankfully not my partner) he did make attempts to try to get her to cheat, he is kept on a far distance for such reasons as agreed between us both mostly her own will, she has never cheated but he was over visiting this morning for the first time in a long time now, and they did a work out together and she was going foe a shower she keeps her clothes on a rack outside our bathroom and the hamper for laundry is right beside our cat knocked open the door a bit cause the stupid knob is broken and door doesn't shut properly she saw him through the Crack in the door just staring at her work out clothes and then he bends down and smells a pair of the underwear she had on yesterday, he leaves after a bit she had errands to run, she told me this when I got home from work now I'm beyond livid inside its creepy as hell and disgusting, every ounce of me wants to just knock him out but it won't solve anything am I wrong for being this mad or just over reacting, she is also disgusted by apology for the long lengthy paragraph

Am I just over reacting or am I legit to be mad?
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