In this case, the writer wanted to refer to either one or the other of the bed partners, not both.

It was one of the reasons he always had an enthusiastic bed partner.

Whenever that happened, he chose his bed partners with care.

It was the first time I'd ever had a bed partner who smelled like hot pork.

This can cause sleep disruption for the individual and also the bed partner.

This can be dangerous to the person with the disorder and his or her bed partner.

She had a right as his bed partner to share in the bourbon.

If you snore, your bed partner also might not get enough sleep.

They had their choice of bed partners- she knew that all too well.

That I was your current bed partner and you weren't quite ready for me to go back to work?