The 'ex's for a reason ' thing to not stay friends doesn't make sense to me because a romantic relationship is a lot different from friendship more things can go wrong where the person is perceived in a negative way. Just because someone is bad in a relationship doesn't mean they wouldn't make a good friend.
The reason that does make sense to not be friends is that it will be hard to due to feelings, everytime you see them you might want to hug/get close to them for example and then when they get into a relationship with someone else it might hurt seeing them with them.
The was a guy I liked in a friendly way and wanted to be friends with, had been for 7 years not close friends but knew and saw each other. The thing is he asked me out and wanted a relationship. I agreed and went on a few dates but was scared of ruining the friendship, I just wanted to be his friend and see him regular like he would with his mates. I told him that I enjoyed his company but didn't want the romance with it. He said it was cool and we still met up etc but one night (happened to be my bday) he said that it was time for him to back off. He said he was attracted to me and wanted a relationship. I ended up going out with him for months as I liked him and didn't want to lose him completely and never see him again. I just wasn't feeling it sexually though, the relationship was on and off. Because we weren't sexually compatable he ended it and texted 'well don't contact me again'. So that was that. I get frustrated that my friends have regular contact with him but obviously I can't talk to him.
I guess if someone is attracted to you or vica versa you just can't be friends with them