I'd slap him too. Not really for that answer, just that Leon is pretty consistently slapable. Also even in context the correct answer isn't, 'I love you both', it's, 'I can't choose between the two of you'. I mean sure that could still get a slap when they are asking him to choose, but one is saying, 'I want both of you' which is actively dismissive of their feelings, as where saying you can't choose while still not complying with the request is a suggestion that you're trying not to hurt their feelings. His delivery was also quite slapable.
Really though what he should have done was take a moment and then stated that if he could choose between the two of them he would have done so well before now, and then offered to let them slap him.