Nope, Derek, you're right insofar as people having children for selfish reasons is concerned. No argument there.
Sadly, even when people KNOW their relationship is in trouble, they insist upon having children, on becoming parents. They tell themselves "When the baby arrives, everything will be perfect". But with all the pressures and stresses of parenthood, it is IMPOSSIBLE to work on a troubled relationship when you now have a child to look after on top of everything else.
The problem really is that people are dishonest, particularly with themselves. They refuse to look inwards and try to find out what is actually wrong in their lives - they live their entire lives on the never-never plan:
WHEN I lose weight, THEN I'll be happy.
WHEN we have kids, THEN we'll be happy.
WHEN I get an increase, THEN I'll be happy.
WHEN I get promoted, THEN I'll be happy.
There is always a "WHEN/THEN" in their lives...
But they don't do it maliciously - they really believe it. They really DO want to be happy - and because they don't feel happy right now, they look around until they find something that they think is causing their unhappiness, and pin all the blame on that. They're actually being quite logical about it, if a trifle unrealistic.
The real thing is this: If you are not happy now, within yourself, then no matter what happens in your life, you won't be happy when that happens. Every change that happens in your life brings stress, because it brings change. Change is stress. You get nothing for nothing in this world - so if you can't cope with your life right now, then you will definitely not cope easily when that new thing (whatever it is) enters your life.
Personally, I tend to blame the 'Princess' syndrome - girl grows up, along comes Prince Charming (who is perfect, naturally), they get married (a perfect wedding, naturally!), they go to live in a castle (which is perfect, naturally!), have children (who are perfect, naturally!), and everybody lives happily ever after. Problem is, NOTHING lasts forever after. Denying it, or pretending it's not so, doesn't change the fact that things always and infallibly come to an end. And NOBODY really has a "Princess" life - you never get everything you want, without working for it, and improving what you have as you go along. Nothing is ever perfect from the start.
However, with regard to the rest of your posting, it's plainly nonsensical to say "if your relationship wasn't perfect, then you should've never had kids". There is NO SUCH THIING AS A PERFECT RELATIONSHIP BETWEEN TWO HUMAN BEINGS!
A relationship is a series of 'exchanges', a compromise on the part of both parties, which eventually results in a win/win situation for both parties.
Anybody who says their relationship is perfect is a liar - or their partner isn't human.
If everybody had to wait until their relationship was perfect before having children, then the human race would be extinct within the next 100 years.
It's not fair (or logical) for you to assume that the OP's marriage was in trouble already when the children arrived. Relationships go through phases - and something that is just peachy keen today is not acceptable tomorrow - as I said before, things change, and people, being the prime movers and initiators of change, cannot escape those changes themselves. Change brings stress - and stress affects relationships.
Perhaps when you are older and wiser, Derek, you will come to understand that your 'absolute' statements as found in your posting, merely point to a youthful inexperience that, with the passage of years and experience, coupled with compassion and understanding for others, will one day bring to you the realisation that you don't REALLY have all the answers.
Don't feel bad, though: We all thought we did, when we were young and inexperienced. We still don't have the answers, but at least now we (more experienced) people understand and accept that we don't know everything, that we can't cure all the world's ills with a single catch-phrase, no matter HOW nice it would be to be able to do so.
There is some truth in what you say - but you need to develop some compassion and understanding, and to realise that you are not a Godlike, infallible being, before what you say will be accepted and acted upon, and thereby have a real and positive impact on other's lives.
I look forward to seeing your intellect tempered with compassion and wisdom over the next few years - all you need now is time to develop it, and the will to help others effectively.
As for the OP's question (How do I avoid being cast as the bitch? ), the answer is simple - behave like a lady, do not badmouth the father, no matter what, and ALWAYS behave as you would like the rest of the world to see you behaving. The rest will take care of itself.