Just for a bit of perspective…
My grandfather (maternal) converted from Lutheran to Catholic to marry my grandmother. But the bigger issue was that he was German and she was Irish - and it was the 1910’s.
My mother converted from Catholic to Congregational when she married my father in the 1950’s. It caused a bit of a ruckus with Mom’s family. She said for her, it was clear - she wanted the whole family to attend the same church.
I married my husband, who is Catholic, and we had our wedding in the Congregational church, co-officiated by my minister and his priest. It was an absolutely beautiful wedding - we chose passages and hymns celebrating the common Christian faith and values. We decided to raise our two boys Catholic, and last year, after much prayer, I decided to convert and become Catholic. I think my mom, if she were alive, would find it humorous that we’ve gone back and forth across the generations!
I don’t think interfaith marriages are the problem with Catholics lapsing. I think that there are lots of reasons why people fall away from the Church. First, I don’t think that many local churches have enough involvement in the community and with promoting family life. My old church had singles groups, bible study, nursery care for little ones during church (VERY helpful with encouraging attendance when you have babies and toddlers!). They had progressive suppers, family retreats, bible camp in the summer, and they didn’t charge for Sunday School. They had a children’s choir, a junior choir, a high school choir, a senior choir (adults), a bell choir (adults & teens) and a string group. Mom’s groups, playgroups, lectures, speakers, and a library.
Now I’ve visited our cousin in Maryland, and her church is so big there that they have five priests! and two churches. They have a lot of the social programs my old church had. My current parish has hardly anything. People want their churches to be part of the core of their family life. They want more than just going to Mass - they want to have a chance to meet others like them, to integrate their faith and practice into their daily life.
Just a few thoughts from an “outsider”