If I were to go purely by the relationship points system, I would say that all the women in the game are main LIs, except for Felvia, Rachel's mom, and Lenny's sister. But as Brannon pointed out, this may not necessarily be the case.
As Buddy in the Incredibles said:
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Same concept applies here.
There are ... let's call it quite the number of girls in the game. If everybody aside 3 or 4 are main LI the story does lose a lot in urgency and drama. The latter maybe returns to a degree in the form of pointless bitching.
For that reason there is a number of main LI's in regular AVN's and a bigger number of side LI's.
Nina, Rachel and Sofia are relatively sure as main LI's.
Other girls may be or may not be.
As said the number of main LI's is relatively limited and that is the reasoning of the question.