When my H first brought up the idea of S, I agreed at first as well... only because I was so exhausted and devestated over everything that I thought maybe it was for the best.
But, like you, I had second thoughts almost immediately. I read some posts from people here that said it is easier to DB when the WAS remains in the home.
So, the next day, I told my H I had changed my mind. I told him that it was my choice to remain M and together in the same home but in separate bedrooms. I told him that if he chose to move out, that was his choice and his alone. I told him it would be his responsibility to tell the kids and that I would tell them this was not my choice.
He was not impressed at first, but could see I meant it.
We had a meeting with the kids in which he told them he was thinking of moving out for awhile. I told the kids this was their dad's decision and not mine, and that I wanted Dad to remain at home.
The kids cried, made it very clear they were not impressed, and would not be ok with a D.
H later told me this had a huge impact on him. When our S15 started to cry, H did too. He then told them he hadn't decided for sure, it was just a thought at this point.
He never moved out.
He broke things off with OW a few weeks laer.
You do not have to agree to a S. But if she chooses to go, you can't stop her.
Stand your ground for what you believe to be best for your family.