
well that i don't know. the whole fertility thing but since we're talking about demons i'm gonna take quess and say its about power. a demon family that has strong heirs (or demon children) are a power house to be reckon with. Verde and Endo come from high ranking demon families and thus, they have to have strong children. Endo was chosen as the heir to his family inheritence and such because he's more powerful then his two brothers. Verde is pretty powerful as well and the two were close since childhood so it was assumed that these two could produce a strong child with a combo of the two. its really about demon status and the power that comes with that status.
Wait, his name is Loup Garou? OMG! HE IS A WEREWOLF! :D I'm so excited!
Oh! And what is the problem with Verde's parents? Why are they so bent on marrying their son? He just got out of an engagement where he totally belived he loved his fiancé! Isn't it a bit too soon?!
Like I said in a comment down here somewhere, if the pattern continues to repeat itself, then Loup Garou will come to Earth to kill Gorou and fall in love on the way. I expect a really cute new uke to show up soon, although being cuter than Hiriki is pretty hard ;)