Riddle

I am just two and two, I am warm, I am cold,
And the parent of numbers that cannot be told.
I am lawful, unlawful--a duty, a fault,
I am often sold dear, good for nothing when bought.
An extraordinary boon, and a matter of course,
And yielded with pleasure--when taken by force.
Alike the delight of the poor and the rich,
Tho' the vulgar is apt to present me his breech.