Pretty much just that...it seems like there would be a lot of little (and not so little) Wolverines running around by now if not. Surely this is addressed somewhere in the comics.
One comic story arc featuring Wolverine had him and his loved ones as the target of a large organization, which sought to destroy him by destroying that which he loved.
He (predictably) slaughtered them all, destroying the organization root and branch. He had the most trouble with their hit squad, a group known as the Mongrels.
It was then revealed to him that this was part of the overall plan - the mastermind behind it knew he'd emerge victorious...and had recruited the Mongrels entirely from his illegitimate offspring.
If he's had enough accidental children to build a superhuman hit squad out of them, it's safe to say he isn't sterile.