I very much disagree.
He wasn't ultimately a bad guy, but the movie paints him as a bad guy the entire time he's on screen.
Because he's fucking Pennywise. That's the joke. This was fresh off his roles in Castle Rock and IT Chapter 2. The trailers even portrayed him as the main villain IIRC. This was all to set up an early twist where he's actually some poor sap bystander who gets pounded into grape jelly. That's it. That's the joke. If you took this very funny flip-flop of "Pennywise the clown is actually not the bad guy he's just some minor character who is the first onscreen kill" as some attack against mankind as a whole, I feel bad for you.
Just because it's painted as a joke doesn't change the obvious messaging. The guy acts like Chris Hemsworth in Ghostbusters 2016. It was played for laughs there too, and everyone said it was sexist and hated it.
Does it suddenly become not sexist because it's Justin Long?
Didn't see Ghostbusters 2016 so can't comment on it. No, the portrayal of a male sex predator is not automatically sexist. Sex predators are overwhelmingly male. Do you think it's racist to portray a gang member as black or Mexican? Do you think it's sexist to portray a "trophy spouse" as female? Sometimes a character can be a bad person and also a man and it's okay. The fact that he's also sort of comic relief takes away some of the ickiness and reprehensible nature so yeah, the fact that it's played as a joke does actually make it even less likely that there's some sort of hidden message here. The hidden message being that most sex predators are men, which everyone and their mother knows.
The actor is Richard Barke. I don't agree with the notion of him being the true villain.
Why not? He's an incestuous rapist and if it weren't for him, there wouldn't be a monster lady living under the house killing people. It's his deliberately malicious actions that led to the characters all being in that situation. I'd say he's definitely not the main antagonist, but he's the villain of the movie 100%.
The movie is a metaphor for diddling
The movie is absolutely not a metaphor for child molestation and I don't know how you could arrive at that conclusion.