Why? Well, for one thing, it's a matter of interstate commerce, something which is explicitly ceded to Congress's control by...what was it? Oh, yeah, the Constitution.
This is something Congress has no business dealing with:
Why? Because it's little more than a cynical, transparent attempt to pander to the Religious Right back at home in the sticks--and what's terrible is that it's likely to work (though polling indicates I might be wrong on that).