I'm going to grab my own wife's butt at a family bbq
What makes you have the impulse to grab your wife's butt at a family bbq?
I love how she looks in that dress.
Most people would expect affection between a couple, but it's highly inappropriate to do something like this at a family event. There are likely kids there and it is not something you should be doing in front of them.
I don't care. I'll do it in front of everyone. What matters is that I like it.
It's fine to be flirty with your wife, but you shouldn't touch her in that way during a family get-together. People are expected to behave politely during those kind of events. I think you should just tell your wife how nice you think she looks, instead.