[quote=DaDrummer676]
Matsuo's Kekkei Genkai would go against his Kaimon. Seeing as it completely destroys the user's body.
[/quote]

[quote=Beta]
The point of the KG is to help the Bushou clan recover from injuries quickly. They are incredibly valuable as warriors due to the rate at which they recover from wounds that would incapacitate or even permanently cripple other ninja. It doesn't go against Gates, it actually pairs with them fantastically.
[/quote]

Essentially what Beta said. I mean, opening a gate is still really risky for them, (see: Matsuo's week long coma) but for the most part, the kekkei genkai is what allows Matsuo to practice the Gates without being out of commission for abundant amounts of time while you guys are out saving the world.

[quote=DaDrummer676]
But it says the more pain the user feels then the more tired they feel. While using Kaimon, the user's body undergoes immense amounts of pain. So wouldn't every time Matsuo uses it, he would fall asleep, or black out?
[/quote]

Lee was able to remain standing and even move quite abtly after opening Kaimon, albeit he was presumably far more used to it then Matsuo would be at this point in the RP, but you're forgetting a little tidbit: some Bushou can fight off the drowsiness with willpower, and I'm sure anyone's get an adrenaline rush from "I opened a Gate and somehow this dude is still alive?" And I wouldn't say Kaimon itself is the one that destroys the user's body, rather puts it under immense strain from the 100% output. But that might just be me.