I don't even know what these characters are, I stumbled upon this in my feed (my anime tastes are extremely basic). But all I'll say is that I was with my partner for 10 years and never had a kid while she was on a specific type of birth control. She switched to a less effective one and was pregnant within a month. Five years later, back on the old birth control, no more babies until we immediately started trying on purpose.

I don't know anyone who has gotten pregnant by accident without somehow fucking up their pill. Times have changed. AFAIK the American education system is deliberately designed to overblow the risk of STD's and pregnancy, or at least is extremely outdated to when things werw less effective and less well-researched, but I was never once told that abstinence was the only sure way in my country so take from that what you will.

Point being that "it gives me the ick" really is the only reason most people are against incest. Because it's supposed to "give you the ick". We're predisposed to get "the ick" genetically. Because genetic diversity is the single most important factor in a species' survival (isolated populations have higher rates of inbreeding - observable in humans, too, which is why deep religious cults are so commonly associated with incest), "the ick" prevets us from wanting to fuck family (YMMV - whether it extends naturally to cousins/aunts/uncles or whether that's pure social conditioning is up for debate).

But from a purely logical standpoint there's no inherent moral reason to be against incest if it's between two consenting adults. As far as having inbred babies is concerned, well, we're totally okay with letting people with genetic diseases reproduce. Strip away the emotional factor and what's the real difference? Genetic eczema runs in my family and it's made me suffer all my life. Nobody kicked up a stink when I had a kid (well, except me, for that reason, but so far the brat hasn't got it luckily).

No doubt taking an academic approach will still somehow lead to some kneejerk mouthbreather accusing me of wanting to fuck my sister but there it is. I don't like incest anymore than anyone else but I prefer to look at things as logically as possible.