Straight up

It's a budget, it should only include budget related things

That's because the headline is slightly misleading. Technically the bill says US government funds can't be used to buy pride flag to fly above state department facilities. Theoretically there wouldn't be anything to stop someone from buying it with out of pocket money and flying it, or use flags they've already purchased. Still ridiculous though, I know.

Our spending bills have gotten absurdly bloated because it's pretty much the only reliable way to actually pass legislation without getting trapped in filibuster land or having it killed by the speaker.

Congress has become a joke and their refusal to do anything effective has resulted in handing over way too much power to the court and executive branch. It breaks the checks and balances and huge amounts of the federal government dysfunction we see is rooted in that.