For me it's definitely an RP choice. I don't always choose one or the other, but in games that give character creation options I tend to go for a quick "non-canon" play test to get a feel for the game and setting and get an idea of how I want to play it. Then I start a new file and create a character to fit that. Sometimes I go for a lithe rogue or a buff fighter, and the gender usually depends on either how I'm feeling or possible story/world stuff that makes it fit better, or sometimes just something interesting. Like in early Cyberpunk there was a glitch where you could start with a male character, then switch some settings and you'd get the female options but it would keep the original genitalia, so I played through as a trans woman because it wasn't something I'd done before and it was interesting and fit well into the setting. It didn't change anything in the game and I kept my character clothed so you never saw her hanging dong. But then I hit the story with the trans woman NPC and my V found a friend who they could connect with a little better. It was a fun role-play opportunity and I felt like it helped my connect to the game and the world even better when my avatar wasn't just a puppet I used to interact with the game. Even in games like the Witcher where you're given a named character, my Geralt always developed his own personality. I once accidentally sold all my boots and didn't realize for a few days that he was running around without shoes. When I noticed it immediately became part of his personality that he doesn't wear shoes. He like feeling the grass when he fights and he's more connected with nature. It kinda fit with the default personality but I leaned heavy into the more nature-focused choices where possible and it changed how I played.

Though I'm probably not an average case-study. I tend to eschew gender norms while identifying as a straight cis guy. I wear what I want, paint my nails or wear makeup if I'm feeling it. And I do lean heavy into the single player RPG games and avoid MOBAs or shooters. I think I've mostly just been playing DnD in all my video games, lol.

Speaking of DnD, my BG3 playthrough started with a female Drow monk because I haven't played any of that in DnD before, but as I played I knew I wanted a rogue so I restarted and as I built it I started with a human male but ended up with a Gith male rogue because I liked the look a bit more for it and knowing what little I did about the Gith in the opening it would be fun RP. But in my head, he's not from a creche but was lost as a small Gith and raised in some small village by human parents. So he doesn't fit in with the Gith he meets but also faces the fear that most people in the world experience when they see him. It just adds so much more depth to the game when they have their own personality.