It seems unfair because it is unfair.
Men spout a lot about how much they love their kids, but when it comes to money, so many absent fathers put their desires before the needs of their children. And then bleat about how they want 50 50 custody. Wankers.
Sorry, no practical help to you, but that's the way the world is at the moment.
My only advice is to try to concentrate on your relationship with your children and to try to rise above the creep who is your ex. Easier said than done, I know.