Ok. Since you're gonna throw out such an enormously dumb blanket statement, give me some examples of lesbians written by gay men OR bisexual women that turned out to be demeaning.
OK. Ryan Murphy. In more than one of his TV shows. In Nip/Tuck, he made lesbian character fall in love and lust with a man. She even married him, but in the end he dumped her. Glee has been called out numerous times too for its handling of lesbians, though as far as I know Santana after suddenly "turning" into lesbian didn't sleep with a guy, though she made numerous sexual remarks to them, as well as comments how hot men are. Ryan Murphy is criticized for his misogyny as well.
Bryan Fuller. He made lesbian character have sex with a guy (and look like she was into it) in Hannibal, though nothing like that happened in original story, and then, used his gay male card to silence criticism.
Carter Covington. He's showrunner of "Faking It", TV show which was advertised as show about girls faking being lesbian for popularity. Despite this offensive-sounding premise, he asked lesbian audience to give this show a chance, that it's not what it looks like, etc. And indeed, it quickly turned out that one of the girls is apparently actual confused lesbian. And in the meantime, near the end, it was revealed by her actress that the character is indeed a lesbian and that she discussed it with showrunner.
And guess what. In the very end she not only slept with a guy and looked like she enjoyed it, but it was also a guy who's life purpose was to 'conquer' (actual quote from the actor) a lesbian. In one interview the showrunner was even asked why the scene looked 'sexy', and his excuse was that the director (who if I'm not mistaken is gay man too BTW) wanted to make it even more sexy, he toned him down.
The last example I'm not perfectly sure of, but I've read that current headwriter of BBC show "EastEnders" is gay man. He introduced confident lesbian character, who even told man who was hitting on her that it won't happen because she's gay, just to make her have sex with him month later.
From the top of my head, aside of Ryan Murphy, all examples are fresh.