I mean, if I were to make a list of positive qualities in a partner "Treats me right" would be the third on that list. Pointing out that she's "the first man she met" isn't a negative here. "Firsts" are important in a solid relationship. If everything with him is her first, and vice-versa, and they treat each other right, that's a rock-solid foundation for a relationship.
She's got the two basic traits needed for a proper relationship: Loyalty, and Gratitude.
Loyalty because she believes his problems are her own: They're in the same boat.
Gratitude because she believes that her life is better thanks to him.
So what if he's all she knows?
The problem is that the girls are literal newborns. Never mind the physiological development mirroring adult female humans, how old is Eliza? A month? A few months at best.
Even if she learns fast, she doesn't know shit about shit. She has no other reference point than what Ichi told her. Even if it was for the best, he isolated them from any other person. His colleague could have been the occasion to address that, but she was a weirdo that saw nothing wrong in their situation, believing they were his nieces yet in love with him.
The girls have good reason to consider him like family because again, I think Ichi did his best to take care of them. He treats them like children that can't be trusted alone because that's what they are, on top of being strange horny and dangerous aliens.
It's the same dynamic irl with children. If they say they love their parents (who are taking care of them well), that's cute but it isn't worth much. To truly love your parents, you need to grow older, not depend on them anymore and actually be able to consider them as a person and not only a parent, you have to know them for what they are and not only for what they do for you and what they represent.
The reverse is also true for (most) parents. It's not hard to love your kids when they're cute toddlers sleeping, it's harder when they grow up to be someone that you wouldn't get along with and frequent otherwise, once they're an actual person to you and not just a glorified pet to hug.
Why I would like to see that even if Cherry wasn't totally happy with Ichi since he refused to 'mate' with her, meeting other humans, she'll see that he was one of the good ones. And even if what she feels for him isn't romantic after all, she'll return to be aligned with her original family while still being free to pursue happiness in whatever way it means for her.
You need two people that are their own person to talk about love. Eliza is still so immature that she throws tantrums when she's frustrated. She should finish alien kindergarten before thinking about a man! 😤