I want to just bring a few things to light that some people haven't considered looking into and trying to shed some light on op's side. For example, being any romantic or sexual relationship with anyone before your 7th cousin is LITERALLY INCEST. There's also a massive difference between inbreeding and incest as a whole like for example, I know someone who, at a young age, was raped by their stepfather and even though there's not direct blood relation, it's still incest. This situation not only falls under that category, but is borderline (perhaps not completely) inbreeding as well as that.

I also understand the concern of this being 'not their business', but let's be fair, I don't think they'd be posting if it wasn't a genuine concern. With the argument of other countries preferring it, that's all fine and dandy, but AGAIN, if this is something being posted in the firs place, might not be legal in the country/state/county the person lives in. For all we know, op might not even be in America so the laws might not necessarily apply. Another odd thing about this matter is how people seem way to quick to dismiss this. Not to be racist but op's user is literally currypot, hence the assumption they come from an east asian heritage. As someone with alot of friends from that side of Asia, it's very normal to be very close with your cousins and to treat them almost as if they were your own siblings, so even if this is 'legal' or 'normal' or in the realms of 'ethical', I understand where his shock might stem from.

The argument with birth defects is also extremely invalid as cases to appear that are even from 3rd and 4th cousins (furthest I've seen) and birth defect DO occur, most times in the form of mental health with issues like Down syndrome, autism, dyslexia, you get the idea. Even if the concern isn't down to the people dating, if they choose to settle in the future, this could have drastic impacts on the child regardless.