It's ALWAYS a bad idea.
Taylor Swift told Vanity Fair that she and her friends don’t care if one of them starts dating another's ex. She says that the girls in her squad are too important to each other to let something like that come between them. It brings up a good question though: Is it ok to date someone if they’re your friend’s ex?
Oh wait, that’s not a terrible question. The answer is very obviously "no." It’s not even that it’s not necessarily OK; it’s that you just shouldn’t do it. There are a ton of people out there, and there are only some that you shouldn’t get involved with. Anyone who used to date someone that you’re currently friends with is on that list.
The problem with doing it is that it puts your friend on the spot.
By dating their ex, you’ve taken an action that your friend now has to respond to. That’s not how you’re supposed to treat your friends.
You never really know all the details about other people’s relationships, and it’s not uncommon for people to have issues dealing with their exes. If you’re willing to even risk putting your friend in that spot, then you probably weren’t ever really their friend in the first place.
It’s the sort of thing you can’t have a rule about. There are times when people are OK with it, and there are other time when they're not OK with it.
How people feel after a relationship ends will depend on a ton of factors: how long it went on, who broke up with who, what the circumstances were. It’s also going to depend on how much time has passed. It’s always going to be super-complicated.
As a rule of thumb, if you know that someone is an ex of one of your friends, then don’t go out with them.
Just keep it simple. There are plenty of fish in the sea, so don’t start swimming with a fish that might ruin a friendship.
Just because someone says they’re OK with it, doesn’t mean they are. (Based on her song writing career, it really doesn’t seem like Taylor Swift's OK with it. It’s probably not a coincidence that Swift has a problem with Katy Perry, who’s dating Swift’s ex, John Mayer.)
This sort of situation can happen when somebody says it doesn't bother them — and maybe they even want to believe that. But in the end, they’ll just get mad about something else you do.
Also, you’re going to put your friend in super-uncomfortable social situations.
Even if they still hang out with their ex, they probably don’t enjoy being around their ex's new girlfriend. And if they don’t hang out with their ex anymore, it’s not going to be fun when you start bringing that person back around.
This is the sort of thing that people like to think that they’re bigger than, but most of us aren’t. Taylor Swift’s interview in Vanity Fair probably has more to do with her trying to prove she’s not a super-jealous person. She knows that she has a reputation for being bad at handling breakups.
But really — her statements about not having problems with exes would be easier to believe in her current single wasn’t "Bad Blood."