I think I have been on autopilot on this, but I vaguely recall, years ago, that there was some media hype about not flushing tampons down the toilet, so I haven't done so since then.
My guess, though, is that water treatment plants screen
I flush mine.
Side question: so what do you do with them if you don't flush them?
If my DH opened up the bathroom trashcan to find a mass of used tampons, he'd have a seizure.
We have a trashcan in the bathroom for such a purpose (along with tissues, dental floss, etc.)
There are certain things men have to deal with if they're going to live with women... and the used tampons/pads are one of 'em. If DH balked at emptying out the bathroom trash containing used tampons, I'd just remind him to be grateful that I don't also use the litter box. :P
Your DH is in for a rude awakening when you have kids...
Now see, if that were my DH, I probably couldn't help myself from torturing him by chasing him around the house with a dirty tampon. Because I'm like that. :)