Because in their culture children are a blessing and having lots of them is the norm.

( Moon Girl Technologies horrendous beanbag, Mon 13 Jun 2011, 18:16, archived)

It's got fuck all to do with "their" culture.
The go forth and multiply creed is almost completely unique to the abrahamic faiths and the bizarre bronze-age sandfucker polytheist warmongers that dreamt it up. Most cultures on earth understand the concept of limiting children to a practical number, even if they didn't have the biological knowledge to adequately apply the idea.
( Dr. Shambolic je suis charlie, Mon 13 Jun 2011, 18:29, archived)

armies require warriors and lots of them

( mongychops, Mon 13 Jun 2011, 18:33, archived)

Prezackerly.
They sort of gloss over that bit in the fluffy Jesus story. There's only one chosen people. The rest of us are grease for their godly bronze swords.
( Dr. Shambolic je suis charlie, Mon 13 Jun 2011, 19:17, archived)