lippylibby - I am married to an atheist. We were both raised LDS. I was baptized Catholic in 2008.
A few things I would consider at the point you are, would be the simple things. I sit alone at Mass, which, most of the time doesn’t bother me because I sit with friends. But once a year, on Christmas, my husband does come with me, and I think how it is that I miss him all the other 52 Sundays, various holy days, stations of the cross, etc.
How are you going to manage this? Are you going to go to Mass while your spouse is at a Mormon sacrament meeting?
The other is, a disparity of faith is difficult. When the disparity is as extreme as it is between Catholic and Mormon, it is going to be even more difficult. Unless, as has already been pointed out, one of you compromise your faith. Is that going to be you? The pressure that will be placed on you by Mormons to convert is very extreme.
Living here in Utah, I know of more than a few mixed marriages that have ended in divorce. A Mormon believes they must be sealed in their temple in order to reach their highest level of heaven. When the fires of love become involved in raising children, struggling with jobs and money, there will come a time for a Mormon who has not been through their temple an additional drive to want this even more. Because Mormons believe that God will only bless people proportionately to how well they are living as Mormons, when times are tough, Mormons will always start to wonder what it is they need to do to gain God’s favor. The lack of a Mormon temple marriage will be a low hanging fruit, so to speak, a very glaring disobedience, to their belief. The pressure for you to convert, at the toughest points in your marriage, will only intensify.
Both faiths are a lifestyle. There are commonalities between the two lifestyles, such as love of family and a tendency towards large families. Both value a chaste life for those who are single, and dating. When it comes to marriage, children, and the raising of those children, both expect and require that marriage is performed according to God’s will. For Mormons, they believe God’s will is a marriage in their temple. This means for you to truly bring happiness to your Mormon spouse, you must convert, turn your back on your Catholic faith, and adopt a new life.
Mormons believe that all children should be raised Mormon. Even if you have the intention to raise your children Catholic, the pressure put on your children by Mormons will be the same as it is for you. I know this for a fact, as I raised my daughter atheist (she is an adult now) and there hasn’t been a time in her life when she hasn’t been targeted by Mormon family and neighbors for conversion.
And last, but not least, I would seriously consider the possibilities that you are leading each other on. A Mormon boy/man has his life laid out before him. He is expected to go on a Mormon mission at 19, return, and marry a Mormon woman in a temple. This is not a suggestion, it is pounded into the heads of Mormon boys from the time they begin their religious instruction at age 3. He knows very well that he is not going to marry someone who is not a Mormon. So unless you are planning to convert, why are you dating him? Unless he is planning to convert to Catholicism, why are you dating him?
I know that last bit may sound harsh, but these are these are the realities for two people, one that is committed to a Catholic faith, and the other to a Mormon faith. If one or both of you are not committed to your faith, that is the one who will compromise. At least for the moment. You have no idea what the future holds, and if/when one or the other of you will be drawn back to the faith you left.