This is one of a series of rabbinic enactments designed to prevent inadvertent physical closeness between a couple while the wife is niddah. They are outlined in the Shulchan Aruch Yoreh Deah 195:1 (based on a statement in the Talmud, Shabbat 13a). In the Shulchan Aruch it states that "he should not sleep with her in a bed". For this reason, most niddah observant couples have two beds that can be moved apart when the wife is niddah.
There are no exceptions to the requirement for separate beds. However, some couples find other arrangements, e.g., a double bed when the wife is tehorah, and a couch, fold away bed or trundle that one spouse uses during niddah.