It's not that surprising. I love Reddit, but it's probably pretty hard to sell legimitate ads on a site that serves hardcore pornography and which is frequented by people who vocally denounce advertising and commercialism in all forms.
You can get your ads served in selected subreddits.
Regarding your second point, you may have to personalize it in order to run a successful campaign on Reddit, but the returns can be good and manifold, especially for a startup.
Yes, there's loads of niche subreddits, some of which are pornographic. However they are in an minority, most people don't use reddit for porn, ergo reddit is not a porno site.
False analyogy. Playboy is a pornography magazine. But reddit is not playboy. Just look at the front page of reddit if you're logged in, no pornography. Create an account, no porn, etc.
If reddit is a pornography site, then so is google.
I was challenging the poor argument of the parent with an extreme example of mixed content. A newspaper, for example, will generally have no hardcore porn in it.
Reddit is a hardcore porn site, it's just it's not exclusively a hardcore porn site. Indeed in part it appears to skirt illegality without blushing. Advertisers do have problems with advertising on such sites I'm sure.
Presumably you and the parent would argue that if you advertised in Playboy next to a feature (I don't actually know the layout, I've just heard they have non-porn articles) then you wouldn't be advertising in a porn magazine?