I was reading up on the Morinth thing, and apparently the amount of Paragon/Renegade points you need for a choice increase as you go on. So the longer I waited, the more points I needed. I think there may also be a bug that includes missions done on a previous import. So if you import a file or do a NG+, it's pretty impossible. In any case, there are people with a nearly maxed out Renegade score that can't do it, and people halfway there that can.

EDIT:

Nvm

Quote Originally Posted by Some dude at the Bioware forums
The Paragon/Renegade system works on a percentage based system. Basically, each dialogue choice in the whole game has a number of Paragon or Renegade points to give you. At each Charm/Intimidate decision point, it then adds up the potential number of Paragon/Renegade points that you could have received by that point, and compares it against the number of points that you actually received. So, say that you could have obtained 100 Renegade points by the time Morinth tries to seduce you, but have only managed to accumulate 65 points out of the required 80 for the decision to be available to you. However, if you had gone truly Renegade on some random dialogue a few missions ago, you might have obtained those points that you need now.