Where is this weird behaviour coming from?
It is coming from the dominance of questions like 'am I being unreasonable?' So, if people think you are being unreasonable saying no, they just ignore what you said and go ahead anyway because they feel that you should have said yes. This thread is a good example of that.
There was a thread on here where a godfather insisted on taking his godchild out once every three weeks (his own schedule) because he thought that was reasonable, and he would not accept the mother saying no because in his opinion, she was being unreasonable. Lots of posters also thought she was being unreasonable.
So what if she's unreasonable? It's ok to say no, reasonable or not. That has to be respected. The only person allowed to argue is the other parent. Everybody else has to accept it (as long as it's not unlawful).
Public Service Announcement: it's not against the law to be unreasonable! You are allowed to follow your own rules in raising your dc and nobody can override that just because they think you are being unreasonable in saying no.
Example, I think it is unreasonable to have a rule that dc are not allowed sweets, ever, even if they're at a birthday party, no cake for them, they have to sit and watch others eat cake while they munch piously on celery sticks. Would I give sweets to a child whose parent has said 'no' just because I think they're being unreasonable? Of course not.
OP, it was your call to say yes or no to your ds being dunked and you don't have to justify that to a random stranger at the pool.