Because they didn't. DM's whole case was built on corporate espionage which never happened.
I know I am talking about the comments in the Livestream, because they were obviously flirting "I found someone to share the rest of my life with "
That makes no sense. Taemoon didn't show up until the very end, and the whole point of of livestream was to clarify that Seonwoo had been fired for doing his job and office corruption, and Mr.Joeng, the nice HR manager, who helped him was being dragged down for no reason. And that DM was disclosing Taemoon's parentage as punishment for helping a fellow colleague, Seonwoo, stand up to the corrupt bosses. And corporate espionage, as DM claimed, was not the issue at all.
Them being a couple didn't even come up until Taemoon showed up towards the end and basically outed himself. So, people who had been tuned in from the start all know why they were fired (both right and wrong reasons) and him and Seonwoo dating is not the issue.
Oh, in case, I wasn't clear (or maybe u didn't get it) the whole argument on Seonwoo's side is that Mr. Joeng is a righteous guy who would helped anyone like he helped Seonwoo because he is a hardworking and responsible guy with integrity. He helped Seonwoo, not because they are dating, but because he can't see a good man getting ruined just for doing his job.
Weird no one said that maybe they wanted to fire them for gay and not for spying