I think that the impact of children born out of wedlock/committed relationships has a devastating impact on our society.
Tell that to my sibling and me. Our dad was a deadbeat, gone by the time I was 5, and we're both happily married, creative, happy, wealthy professionals (and part-time landlords) with no criminal record at all. Our mom was not only single but disabled, and we were so broke that I remember ducking behind the counter so the guy who came to shut off our utilities (this couldn't be done remotely back then) would think no one was home and go away.
I think the key is that the single parent must (1) love the kids and let them know it, verbally, emotionally and practically; (2) teach the kids to value education and aspire to something more; (3) not make a mess of his/her personal life (i.e. not an endless series of shiftless temporary boyfriends/girlfriends), or at least not expose the kids to the mess; (4) not have a victim mindset and (5) cultivate a strong social network, whether extended family or friends who might as well be family, so the kids feel like part of a community and there's someone to pick up the occasional slack.
I'm not negating the importance of a good dad (or for gay couples, second parent) here. But note the adjective "good": having no dad is better than having a bad dad. That said, a good dad is worth his weight in gold, and if I'd had one I highly doubt I would've flailed through my teens and twenties in a series of bad relationships.