I understand the utilitarian perspective you're arguing from: the American Section 8 negro has a much higher standard of living overall than even the Roman aristocrat, because the former has access to a bevy of modern technics including transportation, antibiotics, etc. Roland already addressed essentially why I think it's irrelevant in this case... but to clarify:
The Section 8 negro might have an iPod, flat screen, automatic weapon, indoor heating, etc, that dwarf any of the amenities an ancient aristocrat could hope to possess. But from a relative perspective, he in fact owns
nothing
of real value in his society. He owns neither land nor capital, nor even generally liquid wealth (cash). He is largely invisible to society and serves no valuable purpose. His housing and food is paid for by government and thus places no incentive on him to plan for the future-- ie, his descendants. Promiscuity is the natural result from such a combination. (I'm not saying that negro culture has nothing to do with this, though: it does. But the State greatly exacerbated these tendencies and rendered blacks completely unaccountable)
The German tribesman, on the other hand, owns (and likely built) his humble house, he likely owns (at least in common) some cattle, or farm implements, or some other kind of wealth-generating device. He has an important station in society as a warrior, and a definite incentive to be careful about choosing a partner to procreate with, because his property will be passed down to descendants. As Roland pointed out, siring bastard children is a very quick way for a tribal society to degenerate into anarchic conditions.
To use an example from the ancient world paralleling our situation, Augustus and his successors tried desperately to halt the depopulation of Empire, which was especially acute in the mother countries of Italy and Greece. There were charity food drives, and tax incentives for having children, etc, but short of transferring all de facto parental duties to agents of the State--a task only the managerial state has had the audacity to attempt--there was little that could be done to stop the declining birthrates.
The best solution they could come up with was resettling veterans, usually on provincial farmlands-- in other words, by tying people back to the agrarian life and values in strategic frontiers of the empire. It is also not surprising that most of the veterans who took up this offer tended to be Germans, Thracians, etc, who were not infected so much by the decadent and selfish sexual values of Imperial Rome. Importing immigrants and slave laborers, usually from the rustic Eastern provinces, was the primary method of buttressing the population of the cities.