Can You Date A Friend's Ex?

I'm basically going to be rambling so if you're all about subject/verb agreement and grammatical correctness, then you probably won't like this post. I encourage you to keep reading, but if you choose not to, I understand.
Can you ever date a friend's ex? It's a simple enough question, and depending on who you are, the answer is probably just as simple.  I feel like you have to make your decision on a case by case basis.

Part of me feels like it's okay to date a friend's ex. All single people are fair game, right?  Who am I to tell you that you can't date a guy I used to date? If we're not ever getting back together, have at it.

Another part of me feels like you shouldn't date a friend's ex because it can create drama. It's hard for people to come to terms with sharing the same men/women as their friends. I totally understand that.

Another part of me feels like the length of the former relationship should determine whether or not he/she is off limits. If it's someone your friend dated for like 3 years or more and they didn't have an amicable breakup, you should probably keep it moving. We all have that ONE person that we just really, really don't want our friends to date. I understand that. I certainly have a person who falls into that category. However, is that fair? I'm leaning toward no.

I have this group of homegirls who have all fallen out with one particular girl because she's dating one of the girls exes.  I knew there was some tension between them, but I didn't find out the reason until this weekend. We'll call them Girl X and Girl O. (I didn't know all of them when Girl X was dating the guy, so I thought Girl O had just found her a new man)
Girl X dated this guy for 3 years, about 10 years ago. They didn't have a friendly breakup and they haven't really spoken since then.
Girl O via Facebook, reconnected with him. They started talking and apparently they have a lot of chemistry and etc. She figured it's been ten years, so Girl X would be over it. She figured wrong. I'm not sure how long she had been dating the guy, but they didn't go "Facebook public" until around November of last year. I'm sure she didn't come to this decision lightly. She knew there would be some negative feedback from some people. But, she loves him and at this point she's all in.

As I stated above, I can see this from so many different angles. I honestly understand where both of these women are coming from. But, if you put a gun to my head and made me choose just one side, I'd have to side with Girl O.

 We all know how hard it is to find someone with whom you are compatible. As far as I'm concerned, your dating pool needs to be as big as hell if you want to give yourself an honest chance at finding love.  I know it's hard to separate emotions from these situations, but I'm a little older now and I try to be as rational as possible. If you know you're not ever going to be with a person, what gives you the right to tell them who they can and cannot date? It's basically like you're taking ownership over someone and really, you have no right to do that. I know it can hurt, and yeah pain sucks, but sometimes you have to look at the bigger picture. The thing is, most people have a lot in common with their friends which makes it highly plausible that a guy could be attracted to more than one person in your group.  Each day I'm reminded that as big as the world seems, it really is a small place. I don't know if these two girls will ever get past this. I do know it's unfortunate that pretty much everyone else in the group is upset with her about it. That speaks volumes. She's in a situation where she's potentially lost friends/sorority sisters for good. She deserves a lot of credit for pursuing this relationship when you think about all that she's endured and possibly will endure because of her choice.   I don't know if Girl O and the guy will ride off into the sunset and live happily ever after. I do know, she wants to try, and that's admirable.