I like this VN, and my personal favorite is without any doubt Becca. Lilly easy gets the second place, and her character development is really great. Abby is a sweetheart, but honestly is the less mature of them all.
And the MC... Well... Yeah.. I understand the struggles, but sometimes you have to face reality in stead of running away from it. And the reality is that the girls are not longer kiddies that need an protective wall around them. They have grown up, and the MC should respect that they can, and will, make their own decisions. By trying to refuse their needs and desires, he also shows he cannot accept they have grown up. If he really want to do the "right" thing, he should accept them for what they are. Adults that make that make their own choices. And those choices are choices he has to accept, unless he still wants to treat them as immature brats that have do do what he wants them to do.
Of course I say all these things from a game/VN perspective. What do I want to say with that? Well - You can make a game/VN from a "reality" perspective, and try to make situations like "real life". That only goes so far as a game/VN is enjoyable. If you want to mimic "real life" struggles and decisions, you walk the fine line between "too realistic" and "enjoyable realistic". I think the struggle the MC has with the sex aspect is dangerously close to the "too realistic" side, and too far away from the "enjoyable realistic" side. I think most readers here starting to get fed up with the minimal (or even lack of) progress the MC has. Sure - It's realistic, but this is also a game/VN and not real life. If the struggle goes to a point it is starting to piss people off, the whole purpose of the "message" misses the target. In that case the MC goes from a person that people find sympathetic, to a person that people find pathetic. And that would be a shame, because this game/VN is really nice.
About the whole incest perspective (Abby - Yes Abby is not his real biological daughter, but still very close). Well - Keep in mind it's a game/VN. There are other VN's that also had this theme, and some of them are really great. A prime example is Love Ribbon from Razzart Visual (you MUST plread it with the addon DLC for the best ending), but there are more.
As a (very) side remark - One last thing I would mention is the name of the MC. You see - All people that have seen the (cult) film(s) HellRaiser will feel a bit tormented about the name Frank. It has some strange associations that I cannot fully shake off....... Hehheh - Just kidding.
Anyway - A great update for a great enjoyable game/VN - That's for sure!