Re: "but HOW did daddy put the baby in your tummy?"
We plan on telling our girls (in age appropriate language) exactly as much as they ask us regardless of how old they are at the time and if they haven't started asking by the time they're 6 or so I'll start bringing it up more. Although, at this point my 2 1/2 year old is more comfortable with birth than most adult women I know and frequently requests Gentle Birth Choices (otherwise known at our house as "the brand new baby movie) when it's her turn to pick for movie night so we're probably a little ahead of the curve on sex ed here.
And I wouldn't worry about someone thinking your kid is being sexually abused because they know where babies come from. In fact everything scientific I've ever heard says that a child that knows the right names for body parts, etc. is less likely to be abused, especially on an ongoing basis because they're more likely to know someone is doing something wrong and more likely to tell someone else.