"Perhaps that reflects my viewpoint as a parent rather than yours as a gay? Well, it takes all sorts."
I'd really really like to think this doesn't factor into it, but alas you're right it probably does on some level.
"As I said earlier, I think parents do have a right to bring up children as they choose, even if it's in ways I disagree with."
See, I'm not sure they do. Your children are not your property, and the values you raise them on have a strong impact on society. Referring to extremes again just to be illustrative, I'm really not sure I believe that white supremacists, religious fanatics or the like actually have the right to raise their children as they please. It's not only callous and abusive toward a child, but it's extremely damaging to society. If individuals themselves are sincerely persuaded by the merits of such ideas, then that's a different matter - but realistically a child raised in it might not have the same level of freedom, and the rights of the child come before the rights of a parent.
Now, please please don't think I'm drawing a parallel with what the parents are doing in this case. In this particular case, I don't think the parents involved are necessarily being homophobic (so the wider issue of parents rights is only loosely related). But I do think they're being unreasonable. Particularly considering the script had already been edited seriously - and they appear to have previously agreed to it, so far as I can tell. Not to mention the fact that their children had been working on the project for over a year, and all the money that had been sunk into it. Neither did the school in the early stages suggest an older year group fill the role if they were concerned about the references made in the play.