Yeah, that was something I tried to work my head around. I also tried to think about what would happen if the atmosphere control system- oh, wait, sorry, you don't care about what is and what isn't realistic.
There's a very strong possibility that you would drain all the atmosphere before you fight to avoid the risk of fire. However, I must say that there are so many ways to threaten a fragile space ship and its occupants with death from the outside that there is little reason to board it.
Boarding happened in the age of sail when you wanted what was on board, or it was easier than trying to sink your enemy. It's easy to make up excuses for not wanting to blow up a ship, but in space it's also super easy to get "sunk" and in most cases the risk of being killed should typically outweigh your desire for that VIP or the navigational data or whatever. Not to mention it's almost impossible to board a ship that doesn't want to be boarded, since you can't damage their powerplant for the risk of destroying the entire ship, so when you try to dock using your vessel or smaller vessels, they can just burn at 10 gs to stop you from matching their speed.
Real space battles are usually explosive , laser plasma, firearm free cause chances of depressurising the space boat.
When it comes to firearms, depressurization is not a huge risk. Bullets make very tiny holes, and holes need to be very big before you risk an instantaneous decompression. Imagine putting a dozen 9mm holes in the underside of a tanker. It WILL eventually fill the compromised compartment with water, but it will take ages, and that's only one compartment.
Bullet holes can easily be patched before enough air escapes that people start losing consciousness, unless this was an extended gun battle. Not to mention this is only a significant risk if you are near the skin of the ship. If you're fighting closer to it's spine, there's so much to travel through that a bullet likely won't reach the part of the hull that divides you from space. And if you're the invaders, typically you won't give a shit if the hull is busted since you should be wearing EVA suits.
Not to mention there are firearms that can throw projectiles that will punch through a human's flesh without damaging the wall behind him.