He only just got it on with Lilly in the latest update. After his first time with Becca his conflict was more about bringing Lilly into the relationship than doing more with Becca, though there was some elements of guilt. After he does the deed with Lilly, it is a bit of guilt, though there also worry by the things she said during the act. He knows she has issues and is worried about her mentally. He also really wants them to be a family, when technically Becca and Lilly could easily walk away as they aren't his real daughters (well, Lilly at least, Becca is still a question mark). He is worried that they will change their minds and leave him.
He is also worried about his ex. He got off light in the divorce but she made a claim he sexually abused the girls. While the judge didn't believe her, he is worried if anyone saw them being intimate that would give her fuel to take things further, legally speaking. She could make his life hell, and he knows she is very vindictive. Not to mention worry about how Abby will take finding out he is sexual with her step sisters. These are real feelings a person would have and what helps this story feel more real than something like 23 Sisters, which is a "Go out and fuck and impregnate your sisters. Here's unlimited money" porn game.
His internal struggle makes it feel like a real character and not just an "actor" in a very low budget porn movie. Honestly, if he had no internal conflict and went straight to "Gotta fuck them all", it would be completely out of character. He is a loving step father, that does everything for three girls he isn't related to, but the second he gets intimate with them, he turns off the paternal instincts and becomes a red blooded male that no longer has their best interests at heart? Yeah, no, not in character.
I may be in a minority here, but when it comes to AVNs, I have a "In it for the story" approach. If I want a quick release, that's what porn videos are for. The N in AVN stands for novel, so the story should be engaging and they aren't if your characters don't feel real or do things that go against their established character. Some will say that makes the character sound weak, I say they want a fantasy. That isn't an attack if you or anyone wants the fantasy btw. You do you. I like characters that feel real and this internal conflict makes him feel real. His reasons for it are real, especially for the day after crossing the line with the girls. That said, if he continues to be involved with Becca and Lilly, this should go away quickly, and I hope it does. Though, you can tell it will come back when he gets more involved with Abby. While he isn't her father, he was acting like he was before she was born, so, there are bound to be mental hurdles for him. If there wasn't, the character wouldn't feel real.
