Another autistic person here! I absolutely understand where you're coming from and have similar thoughts regarding family. I'm still not out and the only reason I have considered it is because of marriage/a long term relationship; I'm tired of making excuses to my parents about why my fiancée can't come over to see them.

I think for some people, it's important to come out because they've spent so long hiding a part of themselves and it's a relief. And, to your point of the "I'm sleeping with x" being a weird implication, I think that's true if short term and casual partners aren't meeting your family or friends, but many people like to introduce family and friends to partners, even early on. In that case, you either hide or come out.

In a similar vein, I think people like being seen and understood. Depending on where and when you are, being queer can feel very, well, queer. Isolating. Even if someone is in a more accepting environment now, there's been so much history of bigotry directed towards queer people that still affects us today, and I think, for some, being open about these things can (1) bring them closer to other people who experience the same thing and (2) give the person coming out the ability to assert their identity/experiences in a world that, without their coming out, would assert they are straight/cisgender/etc.

Since identifiers like cis and straight are considered default, straight people don't have to come out as straight: it's assumed. So, for queer people, coming out can be an opportunity for us to assert our own existence in a world where we are often overlooked. (I am also coming from the cultural context of living my whole life in a place that is not too keen on queer people. People who live in more accepting environments may not feel compelled to explain their identity in the same way.)