Single Mommies: No how, no way, never ever even consider this as an option. Single moms are a trap. You already know what her conditions are going to be and you should never entertain them. Leave these women to the AFCs, there are more than enough of them willing to assume the responsibility of her mistake and the guy who helped her with it. No, the kid isn't a mistake, but her becoming pregnant was.
Single motherhood is practically a cottage industry these days. I understand the desire to be her noble savior in exchange for sex, but you have to resist this. Single moms make it all too easy and willingly prostitute themselves for the security a man will afford them. That might seem extreme, but trust me, they are keenly aware of their own conditions and will try as hard as a fat chick to keep a guy and get him to commit. She did it once and she'll do it again.
Now there are the occasional widows of course and you can't operate under these assumptions with them, but you can count on the fact that eventually you will be expected to fill a preconceived role and you will never be her first priority - the child(ren) will be, and this is as it should be really. The energy you will expend in this situation will NEVER pay off when compared to starting a fresh relationship with a childless woman.
I'm only passionate about this because I have numerous friends who I constantly counsel about just this mistake to this day. They started out thinking that they'd be these women's saviors and their sacrifice would be appreciated with good sex, but this was only temporary. After marriage they quickly found out just why she was single to begin with and it wasn't because the previous guy was the 'big jerk' she made him out to be. Single mommies generally aren't the victims they'd like you to think they are. They're not single moms because they had a history of good decision making.
Dont do it.