Yeah, I feel your pain. You would really rather not know about them, and they would really rather you not know about them, I'm sure. Just part of life I guess.
I feel a little different on this but being female on a mostly male only board I can tell you one thing, you need to accept that your current SO has a past just like you do. If they share a child, more than likely you will hear about them off/on for the next 18 years. I know it sucks and sometimes you might not want to hear about them, but it is a part of life. If your SO is content that their ex is doing good in life (or bad) they might be venting to you as a serious change in the child's other parents life could greatly effect how the child is being raised (and with your help and support the SO would be grateful). I know since I am in the same boat.