What do you call bad biological parents?

Anything you like, as long as they can't reach you with that baseball bat.

Seriously, bad biological parents can be *bad*. I've had personal experience of the bad but not-so-horrible type, I merely know people who have been through much worse, and who in turn know people who have been through worse still.

Personally, I like being seen as a "better parent" (see Arienadean's post above), even though I'm clearly not old enough - nor even the right race - to be their biological father, and am not in any position to live with them yet. The best part is I didn't have to do anything special to gain that perspective - they just like me that way better than their real father.

It's a big responsibility though, and you have to make sure you live up to your role, if not their expectations. If you can do that - and if so, it's likely to come naturally - you'll bring happiness to "your" children and to the family as a whole. If not, at best you'll just slip out of the role and become another guy around the house, an uncle, or just a friend.