I absolutely hate that. Hate it when friends try to pressure you to take sides. It makes me retreat, and next thing you know, the friendship with both people is over. It's part of the reason I basically have no friends these days.
In fact, I had a good friend for about ten years. Then her marriage started going sour, and she kept wanting to tell me all kinds of unsavory things about her husband. But I like her husband. He wasn't a close friend like she was, but I still liked him, and didn't want to hear bad things about him. Still, I wanted to be a supportive friend, and it's true that some of the things he did weren't so great.
I think our friendship is over, though, because I was not willing to say, "yes, he's a horrible bastard."
I was going to say that it gets better as you get older, because people mature. I suppose that's true, but it's not 100% true. The friend I lost just turned 50.