Why did he make it a point to tell me he wouldn’t do this?

So I have a friend that I used to have sex with on occasions. We don’t anymore but I was telling our friend group at breakfast one day how a lot of guys I’ve been with end up liking my mom she’s gorgeous so I don’t blame them but still gross! My friend made it a point to say he would never do that. In the mist of everyone laughing. It was mostly a joke so was surprised to hear him say something like that. Especially since we’ve been taking on more a best friend thing then a friends with benefits thing
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Why did he make it a point to tell me he wouldn’t do this?
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