There's usually alot more than that though, the other problem [other than the child] is the "real" father . That's a whole lot of baggage right there, this guy sometimes want to be involved with the kid still and/or the mother , especially if he's paying some sort of alimony / support .
But i'd say the "ease" of accepting the child is most likely based on the guy's previous experiences with kids. I think a guy who didn't have kids before, might struggle a little more getting use to it [not necessarily a jealousy thing, just something he's not use to]. I mean it's one thing going through a whole pregnancy with a girl, being there when the kid is born, going through all the screaming,feeding and crapping and kinda experiencing all the limitations/special things of having a baby....totally another to just being thrown into a relationship with a kid already there and not even sure whether you like the mom, nevermind the kid, without any previous experiences with raising kids etc.
That's why i think alot of single moms tend to go for divorced/single dads instead (imho).