AIO for asking my friend and SO to stop making a cheating joke?

Long story short, I've been with my SO since high school and neither of us have done anything to be suspicious of each other. I don't suspect my SO has ever cheated nor would. But...

I was gone for 6 months for some training several years ago and when I came back, my SO and friend started saying this joke "It was a long 6 months" inferring they were together while I was gone. I saw my other friends laugh at this and tried to be okay and laugh it off but it's always bothered me. It somehow gets brought up enough that I can remember in the past few years it has been brought up a number of times.

Well, today it got brought up again. One of my goals this year has been trying to really just overall be more transparent about my mental health and emotions/feelings and I brought it up to both my SO and friend separately.

I explained to my friend how it just doesnt sit right with me. I told them I never really said anything because I know it's a joke but it's always upset me but I pushed the feeling down because I didn't want to cause a scene or make anyone mad. I also explained that I'm working on my mental health and emotions to give him some context. He said he'd "try" not to make the joke anymore but that I should be able to discern what is and isn't a joke.

My SO is working today and hasn't been able to even respond yet, but am I crazy for even asking this? Am I being too insecure about this? I would never even attempt to make a joke like this to anyone's SO.