...as a Lutheran ignore the Roman Catholics and listen to Martin Luther:
Historical Protestant views on this subject came from reading commentaries on Genesis 38, in which Onan, who married his deceased brother's wife to fulfill his familial obligation, withdrew from her during intercourse rather than impregnate her. God then killed Onan.
Martin Luther (1483 to 1546) - "Onan must have been a malicious and incorrigible scoundrel. This is a most disgraceful sin. It is far more atrocious than incest or adultery. We call it unchastity, yes, a Sodomitic sin. For Onan goes into her; that is, he lies with her and copulates, and when it comes to the point of insemination, spills the semen, lest the woman conceive. Surely at such a time the order of nature established by God in procreation should be followed."
I know you're one of the good Lutherans. You'll do what's right ;-)
What could be more important to discuss ahead of time? Even protestants have traditionally believed that marriage is ordained for the procreation and education of children. So if that is the purpose of your marriage, it would seem that common prudence would tell you to be certain up front that you have some agreement about why you are getting married.
I guess as a Lutheran, I have always found the NFP approach to be a bit disingenuous. It is still a form of birth control, and judging by the fact I don't know to many 10+ families RC, some form is being used. While I know that this is not neccessarly approved, it just seems to be a bit of disbelief in the trenches.
Both of your points are valid. NPF used for birth control is a scandal. But the reality is that virtually no one is actually using it. Nearly all Catholic couples are using artificial birth control. There certainly is a major disjunction between traditional Catholic teaching (and practice) and what we see in the parishes today. Except in traditional Latin Mass parishes. There you will typically see many families of 10+ children.
Remember that your wife is going to be your partner in life, and your partner in creating new souls. You and she have the responsibility in the formation of those souls - your collective goal is to raise children who possess the prayer lives and moral foundations to get themselves into heaven. Don't pass up the opportunity to bring into the world another soul who can bring greater glory to God.
Surely, there will be times in your married life when you will doubt yourselves capable of getting any souls to heaven. It is during these most difficult times that you must try even harder through prayer and sacrifice to let Christ take command of your home.
This next piece of advice may seem harsh but I am saying nothing to you that I haven't said to my own children. If you can't imagine taking advantage of this awesome opportunity to raise up souls who glorify God right at the beginning of your marriage, perhaps you and your fiancee should rethink your choices of life partners.
God Bless and Best Wishes!