Global soccer has major problems with racism and homophobia. Such flags are counter-measures and reminders that the city/club strives for inclusiveness. There's your context.
The pride flag really isn't about sexual orientation anyhow. You're not just gay when you're hooking up, unless you're one of those discreet Grindr people. The original flag's colors meant life, light, spirit, nature, art, etc. The only sex related one was pink and that's not usually shown on the modern pride flag.