It's in there for story reasons, on the first time with a few of the girls (Melissa, Barb, Audrey, and I think a BJ from Sam) but after that if you don't get the points you won't have sex with if you didn't meet conditions. So you're doing something wrong. Use the walk-through because if you don't have the points they'll not sleep with you. You can turn Mary down the whole game. You can not take the BJ and the first time for sex with her you can NOT get her variable. This is all points based stuff.
Also watch on agreeing to threesome's. Again, that's all in the walkthrough. The Mary threesome is in the first chapter there are points for it. Don't get them if you don't want that. But yea other than the first times I mentioned use the walk-through.
Here an example from the code as proof, it's the last time you had a chance to have sex with Sam.
View attachment 2762119
If you didn't get any of these past experiences with her, and you don't have 47 points with her, you don't get the third sex scene with her. You would have had to agree to these things or "qualify" for them in the past in order for this scene to happen. Or else she just thanks you for a nice evening.