Just because Therin wants to mate with human females doesn't make it right...
Actually, that is a whole other can of worms. There is no reason why consenting members of different species shouldn't 'interact'. If they want children, they should adopt... like Madonna! Sarek and Amanda should have adopted a cute Mugato baby. You can't tell me that wouldn't have loads of story potential... sort of like having the Warner Brother's Tasmanian Devil at the science station.
Babylon 5 had a better approach, with the Centauri appearing human but having completely different sex organs and Delenn requiring genetic alteration to make her compatible with a human.
They almost blew it in the pilot episode though, when G'Kar pervs all over Lyta Alexander, hoping to impregnate her naturally.