As a relatively decentralized, herding (redundant, I know) culture, underpopulation represented a significant threat to the Israelites. Especially considering some of their high-powered, centralized, agricultural neighbors. If they didn’t want to become somebody’s bitch, they needed to have enough people to make conquering them not worth the effort.
I don’t think the actual physical act is the determining factor for being less than spiritually sophisticated, but the mental intention of lust behind it. Foundering through lustful thoughts for a release of sexual tension may not be what Gods wants from us. It is hard being human after all in some regards.
Q.E.D.: