Actually in the game when you play as a SW (and I think SI) someone talks about how it's expected for them to have kids. Sith are the top dogs in the Empire, so naturally Sith want to have kids of their own to continue the legacy. Apparently it's expected of them to have children, whether it's in marriage or outside of it. I honestly don't remember what I was doing or where I was when I heard that, but I do remember it being said.
And as far as Jedi go, I think some of them do secretly have kids, just like some of them secretly get married. They just never, ever, ever talk about it.
In the most recent "Old Republic" novel (the one currently in hardcover), one of the main characters is the son of Satele Shan, the Jedi Grand Master. I'm curious to know how that came about without Satele being ejected from the Jedi Order. I doubt it's a secret son, since Satele seems to be a rather straight-arrow type, devoted to the Jedi Order.