It's not the same as parking in a disabled parking space, though, is it. Parking is a zero-sum game. I park there, you can't park there. If I celebrate Pride, that does not stop a lesbian or gay person from celebrating too.
It has become a zero sum game. As I explained earlier, this year there was a limit of 26k people in the parade and not every LGBTQ* person who wanted to march, was able to because so many of those places were taken up by hertosexual people.
Additionally, many charities who work with LGBTQ* people spend a large chunk of their resources they have around raising their visibility at Pride. For example, an LGBT domestic violence charity spent a great deal having cards, lanyards and ribbons printed up, so that the community would know if their existence and have their phone number on hand if they ever needed it.
Their stall was hit by wave upon wave of charming, straight teenage girls who thought that the rainbow ribbons and lanyards were too pretty to leave for those they were intended for. The problem is that in every group of teenagers, there probably was an LGBTQ* youth but it's impossible to single those people, who actually might need help, out.
For a single charity, that's a pain in the arse but when it's multiplied hundreds of times, over so many different charities, it's damaging to the work people are trying to do.
I don't actually want a full blanket ban on straight people. What I'd like is for it to be recognised that a tipping point has been reached when there are so many straight people at Pride, that the event is being damaged.
I'd also like people to recognise that asking them not to use resources not intended for them isn't discriminating against them.