I think if you read it right through it's clear that she's not saying he's gay, she's saying it simply wouldn't matter and that it's a ridiculous thing to think based on him wearing a costume:
'If you think that me allowing my son to be a female character for Halloween is somehow going to ?make? him gay then you are an idiot. Firstly, what a ridiculous concept. Secondly, if my son is gay, OK. I will love him no less.'
and it doesn't sound to me like they're only trying to make her see he might get teased - eg 'And they were that I should never have ?allowed? this and thank God it wasn?t next year when he was in Kindergarten since I would have had to put my foot down and ?forbidden? it.'
IMO parents often use the 'he'll get teased' thing as a cover for their own prejudices - people dress prejudice up as supposed concern for children, but when you look at the actual behaviour of children it's not justified, like when parents complained about Cerrie on CBeebies on the grounds that children would be frightened by her missing arm, when in fact the kids were hardly noticing it, or people used to say (and I imagine some still do) that it was irresponsible to have children as a gay couple because the child would get teased for having two mums or dads.