Sorry, I was looking through the FAQs and I didnt really find what I was looking for. 

I just wanted to ask, is adult content allowed on this site, as long as it's marked as such?

I have a hentai manga style game in mind, but I wanted to make sure I build it here before I went ahead and started

From my understanding, it's a yes for visual novels, as long as it's marked so appropriately. There's an R-18 tag you can add to your visual novel.

Adult content isn't allowed on the forum, though.

yep, i confirm what drakone says