I was right!
There's nothing wrong with using a coupon on a date as long as neither party minds, and if one does and you don't want to piss them off and make the date a giant waste of time, don't use it in front of them.
Also, this is really awkward - don't couples sit opposite each other? But if they did that, we wouldn't see her pissed off expression, so hey.
It's not completely awful, AKA the Lockhorns, because he doesn't seem to be motivated by malice, he's just completely ignorant to his wife's feelings!