Sadly the only NTR games that sell in the west are when they are included as bonus routes, like in Ecaslayer or as RPG games. Neither Marina nor Mariko sold enough to warrant sequels and they were the market feelers. Oyako Rankan has been requested for years as a potential hit but Mangagamer know the numbers better than we do.
The good news is that NTR is still holding up in Japan even as many other genres in their domestic visual novel market are becoming squeezed out. You only need to look at the new releases of major JAV publishers to see how many have NTR slapped on the cover.
Sad to hear that neither Marina or Mariko sold well. Really wished they did and part me of thought they did! Here's hoping that by some miracle they change their mind and release them. Combating piracy is not easy but making them more accessible and available would be a good first step. Hopefully either mangagamer or Fakku will take the initiative and release more games be it NTR or not.
Either way i ask my fellow NTR fans to send emails and spread the word to others to do so as well. If we make enough noise maybe Marina and the mariko sequels might eventually be released. It's a pipe dream at the current stage but then again who knows, sometimes the impossible can happen so why not we all give it a shot and see.
That said the way things are going, it looks like i better start brushing up my Japanese.