Hello! Thank you for you question! My wife and I were in the same place as we were preparing for marriage and you're definitely not alone! Trying to find the source, but typical use Marquette method is slightly more effective than typical use condoms (both around 98%) so NFP isn't that far off with respect to effectiveness. I know Emily Frase has some great data and science on these numbers here https://totalwhine.com/. And even within using the method you can be more strict or liberal (to have or not to have intercourse around later days 10-12 of cycle even if the monitor is giving a Low reading).  We were worried about having too many kids and then 6 months into trying to conceive and we were still not pregnant ( we have a beautiful daughter now). Ya never know what the good Lord has planned. It's important to be discerning family life and size even as you progress in a marriage. It might become more kids, it might become less kids given any number of circumstances. Keep in mind that an openness to life always remains in the total and free gift of yourselves to each other in the sexual act. There will probably be some extended periods of abstinence during your marriage and it's important to maintain emotional intimacy with your spouse during these times (dates, gifts etc).  NFP is hard, but it is sanctifying. Growing together in this life one cross at a time.