Which religious groups? As a Muslim, it's prohibited because God said so, there's really nothing more to be said. The Creator knows what is good and what is bad for us, and we are to Submit to His orders.
In many times, the Wisdom behind such orders is made apparent to us. As we see here (and historically, as per the accounts in the Quran about Prophet Lot Peace be upon him), these groups will not stop at just what's in the bedroom. Today we see indoctrination and outright hostility toward anyone who disagrees with them, causing societal instability.
