I am going to go against the flow (somewhat) and say that this doesn’t strike me necessarily as an ultimatum, so much as her recognition that it wouldn’t be wise to marry someone if you can’t agree on such a basic issue. Honestly, while I disagree with her position on IVF, I appreciate that she is being honest about the fact that this sort of thing would be a deal breaker.
Where I don’t differ with the others is the end result of this dilemma. The IVF problem is only one of many faith-related issues that will be a problem in your marriage. What if you have NO fertility issues, and she refuses to use NFP. Or what if you turn out to be one of those couples for whom NFP is more difficult - will she refuse to make the sacrifices it takes to avoid the sins of birth control and sterilization? Let’s say you did end up using IVF to conceive. Will you always manage to keep it a secret from your children (probably not unless you also keep it a secret from every other person that you know)? Will you be able to teach them to make the right moral decisions even when it’s hard, if you as parents don’t also do that? Are you prepared to have children who are Catholic, but who don’t value the Church’s teachings on things that are difficult?
The thing is, it’s not just IVF and birth control, There are countless moral issues that come up in raising children as well whose decisions are based on the faith of the parents. There are the specific teachings that you would impart about not committing serious sins (you and she probably won’t agree on the sinfulness of these things), and there are matters of what you will allow your children to participate in or be exposed to. These are things that are hard to predict or imagine before being a parent, but a non-religious Catholic and a religious Catholic are likely to have different judgments on these dilemmas over and over again, and this can cause heartache for the parent who has to give in, as well as confusion for the children.
So I would recommend you don’t stop at the IVF discussion, but look seriously at your relationship - not just how much you love each other now, but how your different faith perspectives are going to impact your ability to pass on the most important truths about life to your future children. Remember that the purpose of marriage is to help get your spouse to heaven, and to accept the children that God gives you and help get THEM to heaven. My outsiders perspectives sees the IVF issue as a sign that you should be willing to consider calling off the wedding as well. As horrible as it must be to have to call off a wedding, it can’t be as bad as the heartache of a contentious marriage, and of fearing for your own and your children’s salvation.
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