I didn't think the problem was the marriage inherently?
The thread is about an inherent problem in the marriage - that one person is unhappy and resentful about a decision made by the other.
There are two basic options - accept the decision (and the marriage), reject the decision (and the marriage).
Well, there is a third option, which is where we are now - reject the decision, accept the marriage.
This can take two forms
A resent the decision and live unhappily in the marriage
B take steps to get pregnant (easy if he's expecting you to sort out contraception you don't want) and see if he comes around.
But choice A is what the thread is trying to avoid and risks the marriage.
Choice B has two further sub-options - be open about the fact that you are no longer taking responsibility for preventing a pregnancy you want, or lie and try to get pregnant before he realises that he should have taken responsibility for his own fertility.
Lying to get what you want is relationship poison. Being open about refusing to take responsibility for contraception will most likely mean he will take steps, which is probably better all around, but leaves you back with the first two options:
1 make a choice to stay in a marriage with two children and no more and embrace that
2 make a choice that the way this has been handled indicates a lack of care for what you want out of life, and take steps to end the marriage as amicably as possible
But there is a choice here. He gets to make one choice. She gets to respond with a choice of her own.
Resentment so often comes from feeling trapped by circumstance and having no control over your own life.
Recognising that you have a choice about staying in the marriage doesn't have to mean leaving it. It could well mean choosing to stay.
Which feels a lot different from feeling trapped and resentful.
Telling her she is trapped and must learn to love her prison and stop feeling what she feels is totally pointless.