Also co-signing, though it involved choosing my friends over the course of the last decade and making an effort to stay in touch, my friend circle has a fair amount of parents and non-parents. Paradoxically, I talk to one of my oldest pals more frequently now, because he logs on to a game after his kids go to sleep and is always happy to party up. We are also pretty distanced from one another, but make the effort to check in with each other so we stay close.
Not saying it isn't hard, and not saying it isn't a challenge at times (I certainly don't see my friends in person as much!) but the idea that having kids forces you to be friendless is absurd.