From http://www.askmen.com/women/actress_200/229c_emma_thompson.html
"Unfortunately, her marriage to Kenneth Branagh wasn't experiencing the same kind of success. After she had had enough of Branagh's rumored infidelities with Helena Bonham Carter, the couple filed for divorce in 1994. In 1996, Emma took a sabbatical and started seeing fellow actor Greg Wise, who had played John Willoughby in Sense and Sensibility."
So it was a mixture of both, he was cheating on her and then she left him and went with Wise. No kids involved...she's been married to Wise for a long time and he's married to someone else as well...
Did you see the movie Love, Actually? The scene where she's alone and finds the jewelry her husband bought for someone else (While he gave her a Joni Mitchell CD for Christmas) is absolutely priceless. YOU FEEL HER PAIN. What an actress.