This is the person with whom you reunited, for whatever reason, after previously ending a relationship. He/She was your ex-boy/girlfriend and now is your y-boy/girlfriend. The term was created to facilitate communication. The homonym was appropriate for the reasons everyone asks.
“My y-boyfriend and I are going to his cousin’s wedding.” The listener can understand how that experience may be different than if you were a new, current, or just a friendly date. In general, the listener understands that you have had a relationship with this person, parted with them, grown, and reunited after working out your problems….or not, so some say, “Why?”