@Crazy-San I've also had a problem with pigmen spawning in my portal. I made an unspawnable half-slab area around the portal, but I think they're spawning on the two squares in the portal itself. I suppose some sort of fan setup might work to push them away from the entry, but...hmm. Best would probably be to put a chandelier nearby to prevent spawning.
I guess you could make it so that the portal is on top of a ziggaurat (a step-pyramid) such that any pigs that spawn naturally wander to places where they can't access the portal.
Biggest thing that's helped me is to protect the portal as much as possible and bring a jetpack. ;-)