You are correct. It is a sin. Marrying someone who drinks, murders, commits zina, steals or lies is also not allowed under the exact same principle. Those marriages also are not invalid. The act of marriage earns you sin, the marriage itself goes on. Might want to check how many Sahaba remained married to non-Muslims and for how long. Their marriages would've been annulled had they been invalid.
This is why every kid needs a basic 6 month course in Islamic Jurisprudence.
Anyway, this will be the end of this.