It's not like they grew up together or anything.

Imagine a scenario where you've been in love with someone, who you first met a a total stranger, and are going to marry them, etc.

Nothing wrong with thet, huh? No sick and twisted perversions in the eyes of soceity - two strangers who met, fell in love, and so on...

What if you find out at a later date, both you and your spouse were adopted as babies... and TEH HORRORZ - you discover you were both put up for adoption BY THE SAME PARENTS.

(Apologies for going off topic, but hopefully you can see my point.)