I have three children (younger children). I am at the beginning of getting a divorce. My GID husband and I plan to co-parent in the same home. He moved to our basement guest room over three years ago, and our kids see it as our norm to sleep in different spaces. We have no plans to discuss the details of why we are getting a divorce. The reason is Mom and Dad are no longer able to live as a husband and wife but we still love our family unit. I have no plans to tell my children that their Father is Gay. Kids are wiser than we think. They will eventually figure it out or he'll need to tell them. I would insist that he tell my children if he was not in denial and my children were older.

Confused, you nailed it when you say "I fear not being honest will result in them being angry with ME when they are older."  You've been put in a no-win situation, and you're looking at the least bad option.

You don't say what their ages are -- only that the oldest already knows.  There's a big problem in a family where people keep secrets from one another; it takes a lot of lying and effort to keep a single lie alive.  It's not just that you'd have to explain why you were lying all this time ... you'd also have to explain why you told the eldest but not the next eldest.

When I was 12 and my sister was 14, we had a very close family friend who had developed cancer.  My parents told my sister, but not me.  Eventually my sister and I got into a huge blowout fight and she threw the shocking news in my face.  Many years later, after both of us had moved out of the house, my parents had to put my cat to sleep.  I found out because I attended my sister's baby shower, and her best friend took me aside and told me she was so very sorry to hear about the cat.  

I point this out because lying is never going to be as easy as it seems when you're thinking it's a simple way out of a difficult conversation.  Finding out your family is all a big lie is hard enough, but kids should at least have ONE parent who told the truth.

My children are 16 (girl) (turning 17 next week), 14 (boy), and 12 (girl).
My oldest found out on her own. In fact, she suspected before I did. How stupid am I?
When she initially confronted me about an affair, I didn't believe her. Fast forward 6 months and I found the texts. We were driving 9 hours on a family vacation (without him, of course, be cause he never came on them) and I was trying t to be stoic and not cry in the car. But I guess something about me gave it off and she looked at her boyfriend and said, my dad cheated on my mom. I know it.
When we got down to the beach she confronted me about it and I told her I wasn't ready to talk. So she called my husband and confronted him. He was driving and kept trying to calm her down. When she asked me I felt that it was important to be honest and said yes. This did happen. When she asked who it was I told her it was not my story to tell. She called her father and immediately said the man's name. Her father pulled over on the side of the road and vomited. And so she knew.
She is adamant that the younger two do not need to know. She said my son will never speak to you his father again. And I don't think that comes from homophobia. He actually has a few gay friends. It's just the lies in addition to the other rejection he's felt from him. I do believe my 12-year-old is too young to process this.
In addition to it, my husband does not believe he is gay. So he would be unwilling to admit that that's what was going on.
On top of that the person he had an affair with was an employee and he is a very prominent person in the community. It would be in the papers and it would be awful. I don't want my children to know because I don't want them to have to withstand the public scrutiny of it.  The news would be way too salacious and it would be on every headline for weeks.
My oldest told me the other day that at the end of the day it doesn't really matter who it was with. It matters that it happened. And that he betrayed me. I'm not trying to get her to navigate this ship but I am trying to listen to what she thinks. I've told her that I'm worried that people will be mad at me for not disclosing everything. And we sort of workshopped what we could maybe say.
I think that if I can tell them that Daddy had an affair. And that Mom and Daddy still love each other very much and we plan to co-parent and we plan to be very involved in each other's lives. And if they try to ask who it was with say, one of us will answer that at some point so do not think that we are not open to talking with you about it, but now is not the time for that conversation. Know that you can talk with me about it at some point and that the door is open but now is really not the time for that.
I read in one of the books that kids who find out that parent is gay tend to become very promiscuous trying to prove that they are not gay. And that they have rather unsatisfactory liaison's with the other teenagers and so because they are unsatisfactory they conclude that they must be gay. They didn't enjoy it, so it must not be the right thing for them. And it becomes a very confusing thing. I'd like for them to get through this time of adolescence where they can figure out who they are without the pressures of having this weigh in.
Does anybody agree?

"I read in one of the books that kids who find out that parent is gay tend to become very promiscuous trying to prove that they are not gay. And that they have rather unsatisfactory liaison's with the other teenagers and so because they are unsatisfactory they conclude that they must be gay. They didn't enjoy it, so it must not be the right thing for them. And it becomes a very confusing thing.I read in one of the books that kids who find out that parent is gay tend to become very promiscuous trying to prove that they are not gay. And that they have rather unsatisfactory liaison's with the other teenagers and so because they are unsatisfactory they conclude that they must be gay. They didn't enjoy it, so it must not be the right thing for them. And it becomes a very confusing thing."

I really don't think this is true.  I mean, it's true that you read it ... but I've really seen nothing to support it, as far as kids I've seen going through this in real life.

As Ellexoh says, my opinion is only mine, and only based on my experience with my daughter.  I was pretty concerned at the time, that my daughter knew and was keeping it secret from me.