Alot of people who respond that way, "because it's not in the bible", generally don't have a balance between using the bible as a basis, and using common sense. I've been taught how to reason with the reasonable, and how to recognize those who can't be reasoned with. When possible, I use history science and info people have gained from specifically studying the bible to talk about things.
"Because its in the bible" is a phrase that I leave reserved for extremely bothersome Christians who can't take no for an answer. I've yet to use it.
Not to say there aren't bothersome Christians, I just avoid them.
The bothersome ones that is.