In the Babylon 5 episode Soul Mates, Londo Mollari summons his three hated wives, Timov, Daggair, and Mariel, to attend him at the station. The emperor has granted him permission to divorce all but one of them, and he wants to spend a little time with them all before he makes his choice.
In the course of the episode, it develops Mariel has not only been playing Londo false, but doing so with his worst enemy: Ambassador G’Kar!
As character development, that works deliciously!
Anatomically, however . . .
In the first episode, The Gathering, G’Kar tries to solicit the human telepath Lyta Alexander as a breeder in a program to create a telepathic human-Narn hybrid – the Narn having no telepaths of their own. He offers to be the sperm donor himself, and by natural rather than artificial insemination. “Now, would you prefer to be conscious or unconscious during the procedure? I would prefer conscious, but I don’t know what your pleasure threshold is!” [grins]
So, it appears Narns are sexually compatible with humans.
Centauri, OTOH, have very different . . . attributes. They appear practically identical to humans with their clothes on. But a Centauri male has six sex organs – tentacles extruding from the sides of his body, three on a side; and a female has six corresponding orifices. (Imagine the possibilities! )
So how could any species that is sexually compatible with humans also be sexually compatible with Centauri?!