why only 2 sexes

[SelfishGene]

canalon and skeptic, I know about these general differences you've listed (except for the conjugation between different species) and that is why I said "special unique case of sexual reproduction". Anyway, it is not a argument worth prolonging since we already agree on the differences.

My question now is about the possibility (and probability) of a bacterium performing conjugation with more than one other "mate" before the meiotic split. Is it possible? if so, how probable? Are there any known instances?

[canalon]

There are no meiosis for bacteria. They just split in a simplified mitosis.
I do not know if conjugation with more than one mate simultaneously can happen, probably. But it is not easy to observe considering the size. Also there are a few movies of bacterial porn on the net (look on PLoS one, F. Taddei is one of the authors). They do continue to split during the transfer (hey it can last up to one hour, or 3 generations of E. coli, to complete).
As I said recombination and acquisition of foreign DNA (by conjugation, natural competence, transduction,...) is independant from replication in bacteria.

[JackBean]

bacterial porn I MUST see it!

[canalon]

My bad it was in another minor journal:
http://www.sciencemag.org/cgi/content/a ... /5869/1533
Those are in the supplemental material.