Noun: one that formerly held a specified position or place; especially: a former spouse
As demonstrated in the Webster's definition above, ex is primarily a word used in the past presence. FORMERLY. In reality, it is rather haunting to the present, which is unfortunate; if only words had the ability to stay where they belonged in their proper context.
The past has gone by in time and is no longer existing. I truly believe exes should remain there. In regards to anyone who thinks otherwise, I do not care if you have successfully maintained a "friendship" with your ex. There really is no convincing me. In retrospect, it is rather inconsiderate to your current significant other. Do you want to be in the present or live in the past? Is it more important for you to keep ties with an old flame or focus on the one you have now? If you cannot sever the connection between you and your ex, you need to realize it is for a reason.