Ha ha, very funny…ho ho, it is to laugh…

Personally, I like the whole Angels & Demons spin. In my game they are the least angels of the hierarchy as they are the only ones with enough free-will to do what they want. they come from a separate spacial dimension (instead of space) they have the same number of chromosomes as they take humanoid form. Therefore they can shack up with us human types a procreate like the bunnies with wings that they are.

MSL-CD - 19 March 2010 03:34 PM

The Seraph and Ophanum can interbreed with Humanity for the same reason Elves and Orks can - it makes for a more interesting game and gives the player more options for character races…

Now, if you’re looking for an in-game reason for why they can interbreed, that is a mystery and I’ve posited some theories over on one of the other threads concerning the various genetic reasons (from a sci-fi standpoint) that could lead to such capability. 

In addition, you may wish to consider the episode of Star Trek Next Generation “The Chase” which talks about panspermia, or the “seeding” of genetic data on various planets - this helped explained why there were so many humanoid lifeforms on various planets, and allowed for Spock - a Human-Vulcan example of interbreeding, or B’Elanna, a Human-Klingon example.  If you can accept those, then why can’t you accept Nephilim and similar interbreeding??

I chalk those up to ‘the writers found a funny word and ran too far with it’ like the rest of the technobabble. For AO, I accept the in-game logic that, for some strange reason, DNA is weirdly fluid (you can, after all, buy additional mutations during play, exceed the genetic limitations of your species, and so on), so the ability to interbreed is related to that - the genetic macromolecules are about as stable as ooblik.