User blog comment:DeathGodPrayer/Talk About What You Want!/@comment-80.181.161.74-20121225163512/@comment-5882552-20130127055813

Heh they're devils. They're the embodiment of greed and lust, so a harem is very much accepted. One example is the Bael clan. Sairorg's mother is the daughter of the first wife while Rias' mother is the daughter of the second wife.

As for the second question, I also don't think Issei's soul is of a dragon, just his body. So, maybe Issei's body would never decay since it's a dragon, but his soul is still human-devilish. However, Great Red and Ophis may have done some magic to make Issei have dragon blood as well. Issei's life-death situation is very complicated and mysterious, after all.