My husband is ELCA. I am a cradle Catholic.
It has never been an issue for US (although I know couples who have had problems). As soon as I thought marriage was a possibility (we started dating in college years ago) I began bringing up religion in marriage as a discussion topic. We discussed where we would be married (Catholic Church), how potential children would be raised (Catholic), and the role of religion in the household (Bible studies would be Catholic).
It is not always an easy thing to discuss. People’s feelings get hurt. My husband took some time before we were engaged to speak with his Lutheran pastor (who happens to be married to a Catholic woman with all Catholic kids!) about how he could be the religious head of the household and raise his kids in another faith. The pastor said “Being the religious head of the household is so much more than the religion.” My husband took those words to heart.
He comes with me to Mass every week. We were married in the Catholic Church. Our baby who is due any day now will be raised Catholic as will all future children.
My husband does not have the disdain or hatred for the Catholic Church that some people have, so it makes it much easier. He prefers Catholic Mass to Lutheran service, and he loves the structure and Rome’s role. He just doesn’t always understand certain things like praying to saints & Mary or not letting priests marry.
I should also mention that he is firmly against artificial birth control, and we used NFP until we were ready to try for a baby.
It isn’t impossible to marry outside the faith, but you have to be ready & willing to have some very serious conversations that can get very emotional. Thankfully, we’ve both grown closer to God & each other through this examination, but there are still days after Mass where he’ll say “I don’t understand how you can pray to a saint!”

Usually it ends with us laughing and carrying on about where to go get brunch. We have learned to not get upset with each other when we don’t understand the intricacies of the other’s faith.
I should add that my MIL has a HUGE problem with me being Catholic. The other side of that is that my very devout Catholic parents have no problem with my husband being Lutheran. Unlike my MIL, my husband has NO problem with the Catholic Church. My MIL just doesn’t understand Catholicism so she doesn’t like it. She has a lot of misconceptions.