Even if they have no reason to be in contact, your partner is still sending emotional energy to them. This negative energy can only pollute your relationship. If your partner cannot truly wish their ex well, no matter how much of a pain they are, they are still tied to the old marriage. The opposite of love isn't hate, it's indifference. Too much reaction means your partner isn't really ready to move on with you.
All relationships carry risk. Intimate relationships carry the most because of the trust and vulnerability involved. Going in with your eyes wide open, while not necessarily romantic, is a good strategy for success.
More from YourTango: Does Marriage Mean A Lifetime Of Bad Sex?
Getting involved with someone who has been married isn't a bad thing. Just remember to proceed with caution. After all, those who don't learn from history are often doomed to repeat it.