Can Ex be friends?

Image obtained from http://www.dailymanliness.com/how-to-get-over-a-breakup/

A couple of days back while I got to opportunity to be a judge at one of a toastmaster event. My ex suddenly dropped a message and asked if we could be friends. 

Normally under any circumstances I would have agreed to be his friend. However, this is completely different in a sense that I was already in a serious relationship with my current bf. So my question is, can Ex remain friends? 

As I was younger and slightly immature, I used to think that everyone would be friends even if you have ended things with your other half. However over the years and especially through opinions from my partner and friends, I came to realize that is only possible IF:

1) Both parties outgrew each other

2) Both parties ended it mutually

3) It wasn't a serious relationship 

Having said that, not being friends with your ex doesn't meant you still have feelings for them. It's just you need to respect your life and especially your current relationship. 

There are many different type of friend in this world. Occasionally, you may drop a hi or season greetings with your ex but nothing more. Scrap the talking and meeting on a regular basis because that is never going to happen. 

So for my answer,  NO it is not possible to be friends with your ex whom you once had a serious relationship with. Not only does it abominate your current relationship, but you might complicate your life as well.