Genuine Christians will know that it is a sin knowingly to marry a non-Christian. So if spousal non-Christianity is not an issue, then Christianity isn’t, either.
Sadly there are too many “genuin Christians” that operate on a less than Biblical worldview, still sifting all information through the doctrines of secular humanism, which of course preaches that we are to marry when we “feel” that we are in love.
Maybe they think they are missionaries... :-)
I wouldn’t go so far as to call it a sin. I would avoid relationships with Muslims simply because they cannot be trusted.