You know, I believe that Shatner was married when he filmed "City on the Edge of Forever." Doesn't mean he couldn't pretend to be in love with Joan Collins on-screen.
So why can't Quinto pretend to be in love with Saldana?
I mean, it's not like James Doohan was really Scottish. Or that Walter Koenig was actually from Leningrad.
It's all make-believe, actors playing roles.