Not knowing the characters makes it hard to know how they would react to a 'slight' of any kind.
That being said, any mention of a broken promise during a 'make up', would probably turn into an argument or fight. So that should be avoided at all costs.
Your best bet, at least I think, would be to show one fully engaged and committed to their love making, and to have the other show the slightest sign that they are not equally engaged.
It would have to be something that can easily played off as nothing, like rolling her eyes at a sweet little declaration of love, or maybe letting out a noticeable sigh when she's asked to do something physical.