You're right. This is why I like to loosen the bun a bit; maybe even open a little hole in the bun so I can stick my cock all the way in. Depending on how big the bun is that might mean my cock goes all the way through or I'm up to my balls in hair....which is great.

But yes, you have to be careful sticking it in the first time. The last bun I fucked was a top of the head bun and my cock went all the way through. She said my hard cock made a great hair accessory!