H.,
I was 14 when my parents separated. I can almost guarantee you that they know it is coming.
Just treat them like grown-ups. Respect them to be as honest as you are willing to be with them, and make sure they have their options as to where to live.
I know this is a very hard time, but you are showing your girls how to be happy.
Edit: Geez! I just read some of the responses... I agree with the people who said that you and their dad should be there when you tell them.
I sure do NOT agree with the others who are telling you basically that you should stay with this man who you no longer love! WHAT MESSAGE is THAT sending to your girls?!
Thinking back, there was always open communication between my family after the "divorce". My dad, who was my best buddy, went on this ordeal where he had to create his life all over again, but he still made time for me. My mom had a hard time starting out, as she is the one that moved out. (Odd!) But, she was NEVER happy before they separated. Seeing her living in a little bitty apartment with hardly any furniture or food was hard to see, but she was HAPPY! I hadn't seen her happy in YEARS. My dad went a little crazy - doing things (and women) that he had never done. But, as an adult, I can look back and understand. It wasn't that hard as a child though. I understood THEN even that he was having a rough time.
Bottom line - Do it for yourself. "Broken" families can happen if the parents stay married or get divorced. I don't think it is so much about the marriage - just the quality of happiness that is exposed through the parents.
You be happy, and be the best mom to your kids as I am sure you always have been. They will be fine. You will be fine. You all will move on with your lives and have grown a little from this messy, messy situation.