This is a difficult one. On the one hand, I agree with what many have already said — that one of the outcomes of having children (either through birth or adoption) is that you take on the responsibility of caring for them. In essence, your life is no longer about you.
On the other hand, if you are in such a terrible state that you feel the need to ctb, what kind of life might you be creating for your children? We know that children's development can be affected by so many things — poverty, divorce, unhappy marriages, and even the mental health of the parent. If a parent knew that their child may have a better chance of success (despite the inevitable grieving and emotional/psychological stress) without them, might suicide be justified?
Children will undoubtedly have a terrible time coming to terms with a parent's suicide. And if I'm being honest with myself, it's hard to imagine a situation in which suicide would result in a "net positive" for the child. But perhaps if society had different attitudes towards suicide and a better understanding of why people make the choices they do, the emotional upheaval could be curtailed somewhat. And what if a parent was able to plan accordingly? What if they made arrangements to ensure as smooth as possible a transition? What if they were able to raise their child with a philosophy that enabled the child to process, understand, and accept the choice? That's probably too tall an order, but I'm considering hypotheticals.
These aren't easy answers, and I think it boils down to specific situations. I really try to avoid blanket statements when it comes to judging when a suicide is "acceptable" or not. Each situation is unique, fraught with its own challenges.
I made the choice not to have children because of my mental health issues. I did not want those to affect my child. And if I decided to ctb, I did not want to put them through that.
If I already had kids, it would probably be enough to deter me from suicide. But I'm not in that position, so I can't say for sure. Right now, I stay alive for my husband and my mother, but I know that at some point, my pain is going to overwhelm that sense of obligation. Who's to say what the threshold is for those who have children? How much can/should we expect them to endure? I don't appreciate people putting that judgment on me, so I would not want to put it on anyone else.