Although it's good to know that at least one human being on the planet was willing to date you, when you say "My ex-boyfriend," men simply hear, "The Guy That Used To Bang Me," as in: "The Guy That Used To Bang Me works at Goldman Sachs," or "The Guy That Used To Bang Me is Asian," or "The Guy That Used To Bang Me loves Pinkberry." Say it enough times and we'll wonder why anyone ever wanted to bang you.
If you're talking about him a lot, you're thinking about him a lot, which leads us to believe he probably dumped you and you can't deal with it. People who are ready to date new people, don't sit around talking about old people. More importantly to the male mind, people who are ready to fuck new people, don't sit around talking about old people they fucked.
Even if you feel you MUST mention your ex-boyfriend because he's a crucial character in the story you're telling, there are ways to keep him out of the discussion. For example, you can simply not tell the story that involves your ex-boyfriend. I've gone on dates with women who were divorced, and they never once mentioned the men they were married to for 4, 5, even 10 years. You should be equally as reticent about the Jdate guy you went out with for six weeks before he got bored and stopped sexting you when he was horny.
The only time it IS acceptable to use the phrase "My ex-boyfriend" on a date is when that phrase is followed by "gave me herpes," or "isn't over me and has vowed to ass rape anyone I date." We'll appreciate the heads up. Otherwise, please just stick to the stories about your job, your roommate or your cats. It's a lot easier for us to nod politely and smile when we think we have a shot at becoming the next guy you used to bang.