Speciation of the sexes–what the crap is this?

Ok, I just had a crazy idea come to me like five minutes ago, and since I’m here at my computer, I’m going to write it down.

You know how we sometimes say that men and women are practically members of two different species?  Well, what would an alien race look like if that were actually the case?  What if the males of the species looked so indistinguishably different from the females that, to a human observer, they were two distinct different species?

Of course, this is a question that has been asked many times before.  Just now, though, I had a really weird idea that might lead to a new take on the question: what if the males got pregnant with the males of the species, and the females with the females?

The males would have short lifespans and reproduce very frequently, with very large litters.  They would only have to have sexual contact once to get pregnant.  The females, on the other hand, would have very long lifespans and bear only one offspring at a time, with a very long gestation period.  They would have to have sexual contact multiple times, possibly from multiple partners, in order to acquire the genetic material to reproduce.

But wait?  In this reproductive cycle, who’s the “male” and who’s the “female?” Couldn’t it be the other way around?  Or really, can our human concepts of gender even describe this species–or rather, these two species?

What would a society of these beings look like?  What kinds of gender roles would arise from this kind of a life cycle?  What kind of philosophy would arise from it?  What kind of society?

I haven’t written a lot of science fiction with alien species, but this idea is cool enough that I just might have to do it sometime.  What’s your take on this idea?