When I first started dating nwlinkvxd it was "your mom" and "your dad," but now that it's been awhile, I just call them by their first names. Think about it- if your potential kid had been dating some other kid for a year and a half, would you wanna be known as a mister or by your first name? I'd say Mr/Mrs and Sir/Ma'am is fine for the first few months, but after that, they are people too, ya know!
Or, even better, you could ASK them what they would like to be called.