IMHO there's nothing inherently immoral or unethical when children aren't a factor. They're really the reason you should - (edit) NOT - bang a cousin, but if that's not on the table; it's hardly an exploitative domestic-abuse incest type situation with Child-Parent or Siblings or some sick shit.
I think it's more of just super taboo than unethical or illogical. I'd keep my fucking mouth shut about it if I did though. That said, my cousins are definitely not in contention lol
In a handful of cultures now, but in LOADS of older cultures, it was totally normal. Even expected if trying to keep within a demographic for whatever reason. I think one of the main reasons that changed (and ultimately became a big taboo) is because we learn't about inbreeding and it's potential hazards. So again - if that is not a factor or possible issue/problem- then you're kinda in the clear, honestly - it's your fucking life, if you're not hurting someone else (as having a disabled child would be) then do what you fucking well wont.(edit)