Abstinence is the religious doctrine of refraining from loveual activity until married. To legislate abstinence only education in public schools is unconstitutional.
You could probably claim abstinence is in the best interest of the children, it means they will wait until they are in a commitment with another person so they will be fully prepared if they have a child, on purpose or otherwise. They don't have to come out and say "yeah we promote abstinence because we're christians!"
Besides it does not deny anyone the right to have their own religion, so in reality it does not deny people of the 1st amendment. (Unless there's some religion out there that says kids must forget unmarried before adulthood, but as far as I can tell there is no such thing.)