A woman's male "mistress"...?

Chrystal • With my SO for 9 years & 8 months /married 5 years & 8 months. It is July 2016 & we have been TTC our 1st child since Jan 2015. <3

Is there a word for a married woman's side guy? A married man's side chick is a mistress, but what about the other way around?

I'm a dispatcher & I was trying to put this information in a call for service in a pc way and couldn't think of anything except just "boyfriend"... Anyone have anything they would say?