Unfortunately they don't actually state their reasoning. They just state what they told her while trying to reason with them to go to sleep.
You are probably right, but its just an assumption. In their position I would also be attempting to reason with them by reminding them of their existing relationship instead of "No, I don't want to rape you. You cannot consent."
this isn't right
you have a boyfriend
you have a boyfriend
They don't actually state their reasoning