I have been wondering this question a lot recently. My wife and I always wanted to have kids, from before we met each other. The suddenness with which she embraced a new orientation as gay, and the coldness with which she asked for a divorce, suggest to me that she was waiting until she was done having kids to become who she wanted to be. My kids are 4 and 6. My wife asked for a divorce suddenly and shockingly (she had spent months convincing me that her new orientation included me and we would be together forever). Immediately, she was saying that our family would stay intact, we would remain close confidantes, and still do things together as a family all the time. It would be great and we would tell the kids that Mommy and Daddy mutually decided to live in separate homes and we are so excited. She also said divorce doesn't mess up kids, only the way that parents handle the divorce messes them up. In other words, I needed to get on board with all the decisions she was making in that exact moment, or I would be the one messing up the kids. That week, we lived separately, she looked for a divorce lawyer, and she was never going to look back until our couples therapist convinced us to slow down. I also suspect that she changed course because I was unwilling to tell the kids that this was a mutual decision. I felt it was premature and that I should not take any of the blame since I disapproved.

What do you think about the conversation with my 4 and 6 years when it eventually happens? Is it okay for me to hold the line that we say that she is the one making the decision? It's really important to me to preserve my relationship with my kids, but I also don't want to mess them up with too harsh of a conversation.

Im with Rob. I did not tell my kids anything other than we were divorcing. She went and told them its because "she found out some things about herself and she was gay". She did so without me. She said all of the things she told you, "my new partner will have to like you, we will have dinner together and be best friends, blah blah blah. All done to get me/you to go along so they arent the bad guy. No ability to accept accountability at all. I hope your situation is different, but sounds identical. If you decide not to play along, be prepared for her to go full victim mode. Mine isnt really even gay I dont think, perhaps bi, but being bi would not enable her to escape accountability, so she tells people she is gay. Bottom line, she isnt a good person. 
Wishing a different outcome for you my friend