I prefer it impossible (=lesbian protagonist)
Sadly there are more dicks then lesbians out there and i prefer not to see the mess they make, so pregnancy happens...
Almost always. If a game has multiple endings it can be a better ending to get her pregnant (Melody comes to mind). That is solved by replaying from a save after
Never pull out, but the sad part is that the pregnancy almost never really goes anywhere. I can only think of three games where it actually went somewhere - The Guardian, Rance and Simple Days.