Why didn't Zoidberg die after having sex with the professor when Fry and Leela were in their bodies? Maybe it's mass psychological trauma that causes his people to die after copulation?
They are both males which might make a difference. Also Zoidberg is a lobster and the professor is human, so because the professor is human he technically doesn't have the anatomy to actually allow anything to happen, but it could be a different story if it was with another lobster.
Perhaps it requires two of the lobster people having sex to actually kill themselves, not just one of the lobsters and one of another race. It says in the episode that they abstain from sex until they are ready to reproduce so I think this incident is the first time it's ever happened.
Zoidberg failed to fill the Professor with his male jelly. so, jelly intact, Zoidberg lives on.
Zoidberg's parents appear to be alive after they give birth to him in "A Taste of Freedom" during Zoidberg's flashback, so apparently there is some way for Decapodians to mate without dying.
That, or the writers just forgot about that detail.
Decapodians normaly mate in the water where they also leave their eggs. Without water or a decapodian act of mating, he wouldn't die. It's like humans, he can have sex, but real mating is something totally different.
Zoidberg's body wasn't "in rut" as it were. Since it wasn't mating season, he couldn't have sex in a way that would result in frenzy, since he contained no male jelly.