"I want to fight this" and "I want to dismount" are two separate decisions.
In the case that one must specifically press the dismount button, the penalty for unintentionally pressing the dismount button is that you must mount again. This takes a couple seconds, but can still be done as long as you don't body-pull something while standing there unmounted.
If pressing an offensive action button implied a dismount, the penalty for unintentionally pressing an instant offensive action is that you must fight the thing before you can mount again.
It's therefore reasonable, and quite appropriate, to require two separate button presses to dismount and then attack.