It is odd... I never wanted kids when I was younger. I just never thought of myself as a father (I guess it came from not growing up with one).
When I was 26 my wife got pregnant and before I knew it we had a boy. As much as I would have liked to not have kids... they change your life... I am more than happy that I did.
My son (and now a daughter too) are the part of my life that bring me the most satisfaction. Odd that.
As to the first question... I'd say don't bring it up unless it is asked. If you parents ask you if or when you are going to have kids just say that you don't think you will be having any.
That should be good enough. If they have a problem with this then you will just have to remind them that this is your life and your choice (in a much more diplomatic fashion
)