Not a rabbi but I could play one on TV...
Mystically, in the works of Kabbalah and Chassidut, pro-creation is being a partner with G-d. To take part in this act, akin to creating ‘something from nothing,’ touches the essence of pleasure, so it flows from there that it [is pleasurable, etc....]
Biggirl, from one of your posts (ie, JC and the Church....) you might say, ahem, G-d and the Jewish people.
Re: Go forth.... a little joke: Why is the first commandment given from G-d to man “go forth and multiply?” Because when man saw how fun the first commandment is, he’d want to keep ALL of the commandments! (rim shot, please)