The circumstances are entirely different.
Holly and Stan hit it up at the pub on their own without Lenas intervention (though she has to like Stan and invite him). So Lena has the option to further encourage Holly to socialize with him.
There is no common ground Holly has with Robert and Mike, Ivy setting her up with Mark and Lena with Robert is just a pretense for sex and more or less spell out. Lena is clearly not telling Holly to hang out with Stan because of the great sex he provides.
I suppose the game has sunk in so porn driven that the idea of a normal friendship just became too alien.
umm, why are u blaming the game? you can still keep the nerd Holly and let her flirt with Stan or whoever ever, heck you can also let Lena be innocent. Still, it is YOU the player who can push Holly to change and make her sluty which in turn leads to sexual relationships, which is more in line with her character development. also if this feels too you like porn driven then probably you haven't played GGGB.
For people who say this game is too porn driven, go play "Summer's Gone" which has too much story and romance, and drama with very little sex scenes.