In theory this is supposed to be one of those 'Beauty and the Beast' moments where the MC realizes that if he really cares about Debbie he can't deny her the chance to be happy with someone else. So he stands aside. Debbie, in turn, realizes her love for Lily was illusory and the MC is the real thing. A little trite, perhaps, but well within the tone of the game and quite satisfying.
In practice, though, it runs into some problems. The MC and Debbie really needed to have a serious heart to heart before Debbie starts her seduction run; the quick conversation in the bedroom is way too short and barely touches on the main topic. As a result, what should be an act of selflessness instead feels like Debbie ignoring the MC and/or the MC overreacting after being too slow in bring up his concerns. Not quite as romantic.
Sadly, I think the game was too focused on making sure everyone got to the hot orgy and thus didn't devote the time needed to properly lead up to the moment from all the various possible routes. One can certainly argue that was a prudent use of limited resources, as I think genuinely solo Lily/Debbie routes are a small minority. But I feel like there could have been a better balancing point without significantly more cost if they had focused on the relationships first and the sex second.
Still, this is what we have. IMHO it at least addresses the major concerns in some way, so I'm quite willing to roll with it.