As a director, I prefer for the actors to actually kiss if kissing is required, and I'll try like hell to get it to look like it's supposed to -- sweet, passionate, old married couple kissing, whatever. That said, it depends on what you can get your actors to do. I wanted a really hot ending for the scene in Breaking Legs where the male and female leads finally get together and go off stage, presumably to have sex. But my female lead was very uncomfortable with it, mostly because her husband was working crew and found unexpectedly that he had a problem with it. I had to settle for fake kissing (audience sees back of male lead's head and assumes they're kissing), and a slow dance together off the stage. Fortunately, I had absolutely no problem getting them to do the foot-rubbing bit that ends with looking and sounding like they're having sex. Go figure!

As an actress, it's not me kissing someone I know as a fellow actor, but my character kissing the other character. It's what's called for, and I do it as the character would -- I'd seriously question my acting abilities if I weren't able to do that.

As for marriages ruined by people falling in love or lust with their costars, sure, it happens. People who "get back into theater" after a long absence are especially susceptible to this. More often than not, they choose to get involved both in theater and with someone else because they're already having problems in their relationships. It's been my experience that this doesn't happen with people in stable relationships.