My (34M) wife (33F) keeps unzipping my fly in public and it’s so embarrassing

My (34M) wife (33F) has this very annoying habit where she keeps playing with my fly whenever we are out and about. She will literally undo my fly in public and stick her hand in (but I’m talking with clothes still on just a bit of hip grazing) and I’ve asked her multiple times to stop. She thinks it’s funny and laughs when I try to stop her. It’s seriously embarrassing and I’ve asked her to please stop because other people could notice and that would be uncomfortable. My sister in law told me to just stop wearing zippers and I did and she just then plays with my fly after unbuttoning my jeans. This isn’t even a joke anymore and I’m at my wits end. I’ve tried ignoring her, walking away from her, even humouring her but she won’t stop. She always tells me it’s harmless and that I should learn to take a joke. I just want to enjoy shopping or eating without my wife pulling my pants down and I guess I just need some advice on how to make her stop.<br>Tl;Dr my wife keeps undoing my fly in public and I don’t know how to make her stop