You're kids deserve the truth.  They will respect you for it because they either already know or eventually will.

This is also something I struggle with.  One of my very first posts here included this aspect of things, and boy, did I get an earful from people who said in no uncertain terms that my son (age 27) was an adult and should hear the truth.
  I'm still struggling with this question of if and when I should tell him.  My husband is not gay but a cross-dresser in private who although he would like to live publicly as if he were a woman probably won't come out or alter his body with hormones for some time (he has powerful incentives at work and with his aging father to keep it private).  His sexuality/identity is not other directed, as it is with gay men and women, but at himself as a woman. That changes the dynamic somewhat.  
  If my husband had announced he were gay, I would, because my child is in his mid-20s, probably do as someone else on the forum did, and give him an ultimatum and time window (xx number of months) to do it himself before I did.  I know how damaging it is to all of us to live in our partners' closet; most of us professed profound relief when we found this forum and could finally talk about what had been going on in our lives.  Only in conversation with others going through the same thing do we get perspective, a supportive community, advice, and the opportunity to grow, find our courage, and begin to hope we can and will heal.  So I believe that it's important that we straight spouses should not be held hostage to our partners in their closets.  
  However, I think that all situations are different, and that all advice about whether to tell children has to be listened to with a grain of salt.  Family situation matters; the psychology and developmental stage of each child matters.  Even the gay partner's intentions in the future matter--if your partner has left in a gay affair, the children will know; if the marriage is ending but the gay partner is still in denial, they won't.
  If my children were young, and I were sharing custody, then I think it imperative that they know, as the likelihood their father would eventually enter a relationship with a man is high, and they need to know.  I also think jkpeace is right in saying that for the sake of any of your children who themselves might be questioning their sexuality homosexuality should not be stigmatized.  The problem isn't that the spouse is gay; it's that your sexualities are incompatible, or the partner cheated or stepped out or can't commit to you or to the marriage.  
  Below I've pasted in part of a letter I recently wrote this to my mother (a letter that I haven't yet sent), about my worries about telling--or not telling--my son.  I believe it captures some of the issues relevant to my situation.  I imagine each of us has a version of this.
"If I tell him why I’m ending the marriage, he’ll be thrown into a situation in which he’ll start questioning his own masculinity—and probably our whole past as a family (that’s certainly what happened to me).  If I don’t tell him, will he blame me for the divorce?  If we simply tell him it was a joint decision, what effect will that have on his own ideas of relationships, or the way he conducts himself in a relationship?  If I don’t tell him and he finds out in the future, will he blame me then for not telling him earlier?  How might his girlfriend—or whatever woman he’s with at the time--react if/when she hears about this?"
   
   

Thank you for of all the replies. I do believe they deserve the truth. They've witnessed a false marriage for most of their lives - they should know why their parents didn't display a loving supportive relationship. I'm so tired of lies. I just don't want to damage my son more. My daughter lives in NYC and is in the arts. She will be able to deal with this much more easily than my son who lives and works with my husband in our small, conservative town. He will not believe it. My husband, like many of yours I'm sure, does not in any way fit the stereotype of a gay man. Sometimes I think it would be easier just to say he had an affair and leave it at that. So confused and conflicted.

It may be *his* secret, but it's her life, too.  And this is a very real, very big part of it.  Just because they don't want to live authentically doesn't mean I don't want to.  If MY story is "my ex is gay", then I don't feel it's purely his story to tell.  It's mine, too.  I'll respect his decision not to tell his own family (whom I never see anyway) - that's fine.  That's his secret to keep from the people in his life that aren't in mine.  But there is no way I'm staying in the closet with him.  I never wanted to be there in the first place.

Kel