I know this scenario so well. I have friends that are both.
I'd say that the first one is more responsible b/c they could be trying to get their money together and it's not absolutely necessary for them to have their own place. The "YET" makes all the difference. If they're making an effort to save, it's a no-brainer.
The second is a bit more irresponsible b/c being a COUPLE and living on your own means they made a conscious decision to do it knowing that they BOTH couldn't afford it.
Personally, I don't think you should decide to live with a significant other if you can't afford to live by yourself.
Kind of undecided for the single parent... :notsure: My friends are a couple. Had a kid unexpectedly and they're always asking for favors, money, and help. I'm working on getting my own shyt together so I've been ignoring their calls lately.