This report says that 82% of football-game-going people "would not have a problem" if a gay player was on their team. 8% would abandon their club.
That maths suggests as much as 10% would turn up and think homophobic abuse toward their own team was the right thing to do. Manchester United's ground has a 75,000 capacity apparently: if half of them are supporting the home team and one in ten of those are good with yelling (whatever), that's nearly 4,000 voices from your own fan group telling you to **** off for being gay. What a welcoming environment.
And we don't get the more meaningful questions - if the other team had a gay player would you incorporate that into the degrading shit you yell at the other team? And if someone next to you was shouting homophobic abuse, would you tell them to STFU or try to report them for a hate incident, or let it go because it's banter against the opposition?
My understanding of football crowds is coloured by the ones I've encountered in the street as someone who doesn't look like "one of us". It also comes from the one game I attended: Manchester United v Red Star Belgrade in 1991, where the UK fans were yelling abuse around the slide into war that was happening all around the Serbian team back home. Your homeland is imploding and you're about to endure years of bloodshed and terror: but you're on the other team so we think that's fair game. What lulz. I didn't go back.
It's 25 years on. I am not convinced the attitude and behaviour on the terraces has evolved since then, though I'd be glad if it has.