It's a way of telling that the vacuum seal on the jar hasn't been broken. If you push on the top and it doesn't make a popping sound, then the bottle hasn't been opened after it's been sealed and whatever is inside is still good. If you push and you hear a pop, then the seal has been broken and you probably don't want to consume whatever is inside.
In the context of the comic, I'm pretty sure it's insinuating that the girl isn't a virgin and has possibly been sleeping around/cheating on the guy before they were married, which would have been scandalous had it taken place in the Victorian time that the comic is set in..
I have so many questions... best not to ask.
So... He doesn't like caps that pop?
It's a way of telling that the vacuum seal on the jar hasn't been broken. If you push on the top and it doesn't make a popping sound, then the bottle hasn't been opened after it's been sealed and whatever is inside is still good. If you push and you hear a pop, then the seal has been broken and you probably don't want to consume whatever is inside.
In the context of the comic, I'm pretty sure it's insinuating that the girl isn't a virgin and has possibly been sleeping around/cheating on the guy before they were married, which would have been scandalous had it taken place in the Victorian time that the comic is set in..
A+ explanation.