They worded it that way to find out if it's bias coming from your previous ties with girl scouts or if it's just your actual general opinion. And the fact that you consider it to be sexualizing children, but are apparently okay with some instances of sexualizing children is the point that they were trying to get at. It's not about "winning an internet argument" it's pointing out a double standard that has resulted in you explaining that you have a criteria for when sexualizing minors is and isn't acceptable. He was right to do so because at least IMO what you're saying is incredibly sus despite on the surface sounding like it's protective of children.
To me, and I assume the majority of other sane adults, it's pretty clear when an adult is doing something taboo for the sake of the taboo and can be appropriately viewed in that context. It's one thing to find it distasteful or off-putting or whatever, but to go as far as saying that it sexualizes children just makes me wonder why that's even a possible consequence of something done between two adults. Like I already know I'm not the type to ever harm a child, and there's no amount of cosplay that will ever have even a hint of an impact on that.
And to speak more generally on the interest in the cosplay in the first place, it's really not that weird when you consider how everyone was a student at some point and has memories of things they did or wanted to do but weren't able to for whatever reason. Probably most people into these cosplays approach it from that place of nostalgia/acting out missed opportunity, and with that in mind I think the vast majority of people would consider calling people who have fond memories of teenage sexual encounters "pedophiles" to be a pretty extreme over-reaction.