Strange bedfellows

“Strange bedfellows” is a metaphor to describe an unusual alliance or relationship, especially between parties that normally have opposing or very different viewpoints.

“Bedfellows” refers to people who share a bed. Sharing a bed with someone is a very personal, intimate act, typically reserved for those with a close relationship; therefore, if you have “strange bedfellows,” you’re in a pairing or relationship that is unlikely.
For example:

• The right-wing radio host Sean Hannity and the left-wing senator Ed Markey are strange bedfellows in the fight to save AM radio. (They don’t normally agree on much, but they agree on this issue.)
• The environmental legislation has made strange bedfellows of oil companies and green activists, both supporting the new regulations for different reasons.