Your teacher's "dumb excuse" originates from Paul.
2 Corinthians 6:14
Do not be unequally yoked with unbelievers. For what partnership has righteousness with lawlessness? Or what fellowship has light with darkness?
Paul's advice sounds harsh to modern ears. But it's sound, practical advice.
A non-Christian spouse will be lukewarm or even hostile to Christianity. S/he might encourage you to "tone down" the Christianity, especially in front of the kids. In time, you'll be lukewarm, and might even leave the faith.
I've heard several former Christians call Dennis Prager and tell them that they're converting to Judaism because they're marrying a Jew -- and because Prager convinced that that religion is about "values," so it doesn't matter which faith tradition you follow, so long at it has "good values."
Mixed marriages also fail to properly spiritually instruct the kids. They usually end up as “nones.”
Well...he went nuts. No shrimp in his house!!
Took my shrimp and my clothes and moved out that same day. No one tells me what I can eat or not eat. I'm not Jewish...he was.
“... In time, you’ll be lukewarm…,
See Olivia Langdon, Mark Twain’s wife.