Most people don't consciously register whether they put their right or left leg in their pants first, but they do it the same way every time.

Both simultaneously gang

I put my right leg in, I put my right leg out, I put my right leg in and I shake it all about. I do the Hokey Pokey and I turn my self around. That's what it's all about.

Edit: Auto correct said Hokey Pokémon. I really wanted to leave it...

Next time I'm at the gym I'm going to be very weird looking at everyone putting their pants on.

This is your fault. I will not thank you for your service

Damn you for making me be conscious of this now.

Damn you to hell!

(Upvote granted)

I have a trapped nerve in my spine, so I'm well aware of my method as other methods goose me.

Left then right. I've been this way as long as I can remember. Doing it the other way is simply odd.

Does nobody else just do both at once?

I have alternated over the years due to having various injuries and it's made me change how I do it. Socks however have never changed.