I think I'm 50/50 with this one. On one hand, he's allowed his opinion. He should be allowed to promote said opinion publicly, as free speech should allow this. And the arrest was unnecessary - police would be better suited to actual crimes. I would have expected security to remove him.
However, whether you like or dislike trans people, whether you believe they have a mental illness or not, going to an event where a lot of trans people attend and feel safe, happy, even excited - just to cause upset is quite poor behaviour.
Whether you believe it should be LGBT or LGB is again, your own opinion. For now, pride is LGBT friendly. I went to my first pride when my eldest (14) was a year old and it was LGBT then. I attended a support group years before that which was LGBT friendly.
Trans people aren't going anywhere. GC people aren't going anywhere. So we can either spend our lives arguing and fighting each other, having that hatred and anger passed down to our children and grandchildren, or we can find a way to live amongst each other amicably. I am not including men pretending to be women who just want to rape women in this.
Unfortunately, nobody seems to want to move on either side so we'll just continue like this for God knows how long. I doubt I'll see what happens in my lifetime, but it would be nice to think that we could sort this whole mess out.
Just to add - a lot of things we're put down to ask "mental illness" years ago which are now considered perfectly normal.