"I don't know - we're kissing a lot and I was always told kissing led to babies. What are we doing wrong?"

Go on - just for the look on the face and see everyone stop asking within days.
That's a classic! lol

OK, I have personally used these following lines (a couple of them didn't go down well, but hey, these people were rude with their prodding into my personal life!)

Well, we have, like, sex 5 times a day, so I don't really know why it's not happening, and like, we try different positions (go into great detail here hehehehehe) so really I just DON'T KNOW

It's a work in progress

We'll have them when we want them

Stop being rude and mind your own business

If the person asking is a tad overweight, give your answer and then ask 'and when will you lose a couple of kilos' and the reverse if they are on the skinny side.