I ask about exes, but not because I want them cut off
So whenever things appear to get serious with someone I'm dating, I ask if they spend time with any of their exes.
In an ideal world, they would volunteer this info (as i do, as I do in fact stay in touch with an ex) but I have learned that it's better to take a proactive stance on these kind of things.
Anyways, being in touch with an ex isn't inherently a deal breaker for me, but it can be. A prime example is when I found out they still LIVED with their ex, and they only broke up like 3 months.
So what did I do? I broke things off. I did not demand them to break things off with their ex, I simply moved on.
I would have done the same thing if I felt like their relationship with their ex was something I simply didn't want to deal with.
My personal philosophy is to either take people as they are or walk away.