...right now. It's commonly--though by no means universally--believed that this will change, and that we've already seen other characters traveling by other amounts of time. Consider both ninjas, for example. So I don't think this precludes the chance of MC at some point in his future going back to, say, late 1998 or early 1999 and getting Tiffany pregnant. I don't think that's what happened--from everything I've seen, I think Jessica is Eric's kid--but I do expect we'll see more flexibility in his time travels, which would allow for it.
With the current timing we've seen, the only one MC could have fathered would be Skylar.