Jews don't condone sex with a concubine, why?

The only thing we (jews) don't allow (if you're seriously into religion), is sex with a concubine. Why is this?

To be fair, while we do allow everything else, it doesn't necessarily mean we'll be happy with it all. It's just that that's the only thing we really just don't allow

Jews don't condone sex with a concubine, why?
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