One of the first rules we learn about dating is that our friends' ex-significant others are strictly off limits. And following that rule would be easy if our friends only ever dated unattractive people, or if once the relationship ended our friends' significant others disappeared out of our lives forever.
But at a school the size of Geneseo, that's just not likely. At one point in our time here, it's highly probable that each of us will be interested in someone that one of our friends once liked/hooked up with/dated/considered the love of their life. If your circle of friends is large enough (or promiscuous enough) you could find yourself dating someone that more than one of friends has been involved with.
Conventional dating wisdom says that if we act on our attraction, it will ruin the relationship we have with our friend. Boyfriends and girlfriends come and go, but friends are forever, right?
Well, maybe not. Think for a minute about how many of your friends from high school you are still close to. Now think about how many of your friends from elementary school you still even talk to. Throughout our lives, we make (and lose) a lot of friends. Some of those friendships will last a lifetime. Most won't.
That certainly doesn't mean that it's automatically acceptable to go after a friend's ex. Just because you might lose touch with someone later in life doesn't mean you should deliberately hurt them now. But you don't have to give up on a chance to make yourself happy because it might cause someone else a little pain. Each situation is unique, and it deserves careful consideration rather than blind acceptable of rules.
How you should handle yourself in a situation involving your friend and their ex is complicated, because as common as these situations are, no two situations are the same. The decision about whether or not to make out at the IB with the guy your roommate broke up with last week is not the same as the decision about whether or not to start dating the girl whom your lab partner has a crush on. And neither one of those situations is the same as realizing that you are seriously falling for the person your teammate was casually seeing last year.
Before you act, think about your friendship with the person who might be hurt. Think about how much it would matter to them if you dated their ex. Think about where your friend and their ex stand, and how bad the breakup was. Think about how much possibility your potential relationship has. Maybe most importantly, think about what decision you would regret the most (or least) if things didn't work out the way you hoped.
Friendship is important, but so is making yourself happy. The two usually aren't mutually exclusive, but when they are, you owe it to yourself to stop and think about what is really the best choice for you. Most of the time that choice is going to be to save the friendship. But if you follow the rules instead of at least considering the possibilities, you'll never know what you could be missing out on.