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tempsventeux: Ryuu would definitely be a better option than the kid growing up without a father.
And while I do agree that there a children that "grow up just fine" with a single parent, this is the exception rather than the rule. I really hate to link wikipedia, but I don't think anybody would bother reading if I link a paper, so here: https://en.m.wikipedia.org/wiki/Father_absence
She's simply making a choice that increases the probability of ruining her and her child's life. The worse thing is that it's not like she has no options. If you've watched Hotaru no Haka you can see the exact same thing happening - a main character making a bad choice due to lack of information and due to his ego. It's tragic seeing characters make decisions which may ruin their and their loved ones' lives.
But I guess this is what actually makes the plot good - a pinch of realism. Even though I don't think the author intended for this. Also there's no tragedy tag, so I am assuming the author will try and make it work out somehow.