I am a 40-year-old woman; I came out when I was 16. When I was 17, I met M and we dated for eight years. M was a horrible human being—emotionally and occasionally physically abusive. M still sends me the occasional (creepy) e-mail, wishing me a happy birthday or giving me updates on people I don't really recall. I don't respond. A few years back, I got an e-mail saying that M was now "Mike." I think it's important to use the pronouns people want you to use for them. But Mike wasn't Mike when he was in my life. Changing his pronoun when describing him feels like I'm changing my identity—my first real long-term relationship was with someone I thought was a woman. Mike caused a lot of damage in my life—does he get to fuck up (or complicate) my identity, too? It's not like the subject of Mike comes up daily. When it does, I feel like a liar if I use "she," using "he" makes me feel like I'm lying about myself, and stopping to explain everything derails the conversation. And it's not like I'm being a great trans ally when a conversation gets sidelined by something like: "Well, random coworker whose only trans reference is Caitlyn Jenner, my ex is trans and he's a psychopath."
—Mike's Hard Lemonade
Block Mike's number, block his e-mail address, block him on Twitter, Facebook, Instagram, Snapchat, LinkedIn, Periscope, Kik, FuckStick, WhatsApp, CumDump, etc., etc., etc., ad infinitum.
And stop talking about Mike—don't discuss him with random coworkers, casual acquaintances, or friends. If you absolutely, positively must discuss him with someone—a true intimate with a right to your relationship history, who needs to be sensitive to the abuse you suffered—you can be a good ally to other trans people (not your abusive trans ex) by carefully using nouns and descriptors in place of your asshole ex's preferred pronouns. So instead of "I met him when I was still a teenager," you say, "I met the abusive piece of shit when I was still a teenager." Instead of "It took me eight long years to get away from him," you say, "It took me eight long years to get away from that asshole psychopath."
What I'm gonna say next will get me slammed on Twitter (heavens), MHL, but I've learned not to read my @s, so here we go...
If using male pronouns when referring to your ex is gonna complicate your life—really complicate it—if the "transitioned later" part is likely to get dropped during a game of interoffice telephone, if the qualifier about your ex having identified as a woman while you were together is likely to get dropped too, and if either of those drops could lead coworkers or casual acquaintances to assume something about you that isn't true, i.e., that you're into dudes and therefore gettable by dudes, and if that erroneous assumption could result in your having to deflect awkward and/or unpleasant advances from confused males, or if having your status as a Gold Star Lesbian questioned could induce orientational dysphoria... I don't see the nontheoretical harm in you—and only you—misgendering Mike on the rare occasion when a convo about him can't be avoided. You don't live near him, no one you know knows him, and the misgendering is unlikely to get back to him. The adage "no harm, no foul" applies here.
But it would be simpler, easier, and ally-ier if you sidestepped the issue by not speaking to anyone about your asshole ex ever again.