Actually, tits aren't pornography themselves, but staring at a variety of tits is pornography. It's the act of staring and focusing intently getting aroused at staring at the visual object that makes the subject pornography.
Definition - Pornography: printed or visual material containing the explicit description.
or
Display of sexual organs or activity, intended to stimulate erotic rather than aesthetic or emotional feelings.
If you take the first definition, Computer circuit porn could be staring at circuit boards of all types.
So while tits are posted on this site, it's the seeking out and staring at the many variety of visual material, but it's not limited to just tits, it can be anything that fits in that category.