I've posted before about how I feel the writers don't much choice here. You can't have a sweet innocent character that has sexual attraction to her father and not also be naive as fuck. Playing "move the goalposts" about sexual subjects in about the only sane way to explain why she's slowly ok with the increasingly perverse acts she's committing. I've personally known people that claim shit like "anal sex isn't really sex, so I'm still a virgin" or "blowjobs don't count as cheating." Shit is more common that you'd expect.
As to her being unbelievably naive and innocent, well those kind of people really do still exist. She's refused to talk about, look at or watch anything of an adult nature up the the point of the game's story. Sure Elena is a big old slut, but that doesn't mean she hasn't respected D's wishes to not expose her to that stuff. As D seems to be pretty unpopular in school I'd say the odds of other kids talking to her about sex was pretty low. Yeah she knew what Lucas did was wrong, she knew it was wrong with her dad too and she said so. Difference is she didn't feel anything for Lucas. Deep down she's had feelings for her dad from the get go, and didn't know how to deal with them.
Yeah a lot of the mental gymnastics the characters are going through are rather bullshit to an outside observer. Most peoples hypocritical justifications are in real life too. Doesn't mean they don't make complete sense to the person that is using them. Especially when their emotions are making them want something bad, and they have to convince themselves there's a logical excuse to make it ok.
You want a good counter example of what you're seeing in this game, go play
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. The MC there is a former drug addict who has cleaned up his act. By the end of day one his estranged wife who has only talked to him online for months is already boning him and talking about moving him back in. By day 2 his oldest daughter is trying to get in his pants. Mother catches wind of this and is amazingly ok with it immediately. Few days later and his youngest daughter is in line too. No ramp up to getting to know them, no need to build any trust. It's fun and erotic, but from a story perspective the narrative is jarring and strains belief.
I'll take "Blue Balls Simulator" to "Incest Porn Tropes" anyday. At least that way I give a shit about the characters having sex by the time it happens.