Is it normal for my girlfriend to casually bring up "if we broke up..."?

As both of my clones said, you should start looking for a new partner. This girl is already dismantling the relationship in her head and there is nothing you can do.

I think it's normal.

Why not just talk to her about it?

Be ready next time she brings it up. Let her know in detail how you would move on and whom you would do. She may just be manipulating you.

You really need advice on this? sounds like you need new tits to touch.

I guess that depends on how often she says it, and what she follows it with.

As three of my clones already mentioned, find a new girl ASAP.

I think she's testing you. It's truly a no win situation. As my clone says; you should be looking for your next ex, because this one is gonna be your present ex real soon.