Not sure this happens with you ladies. I think it might be a guy thing. It might even be considered a male trait. We can't help it. It's a mindless activity. Half the time we aren't even aware we're doing it.
Watching TV, reading a book, lounging around, soaking in a tub, it happens. The hand moves to the crotch. The pleasurable sensations cause the cock to respond. But there still is no thought beyond that. There's no real intention behind it.
There could even be some mindless stroking of the boner; mindless masturbation. It's a thing.
And, when we become aware of the boner,
sometimes it turns to deliberate masturbation
sometimes it doesn't
How often have you found yourself in mindless masturbation?