More than one. There have been at least two points, and if I remember well, three points where you could cut it, and having sex with Marc was never mandatory to start with.
The last of these cutting points is the pool scene at the new house, where Myriam can decide to stop doing stuff with Marc.
It is true that in one case, having the incest patch, it is possible to to say no, but then have Myriam still go to watch online "incest" porn, which would leave the door open to the possibility of going anyway back in the next update (since it will contain a scene with Marc). And if I remember correctly, before that, at the pool, Myriam can say "we cut", but then do a "last time" with Marc, which, especially if eventually joined with the "incest" porn, could give some doubts about whether it is really the last time, or rather "the last time that precedes the next last time'
.It will depend on how I can play with it, without changing the main line of the story, but that in principle should not be a major problem, since Zorlun has anyway already foreseen the possibility of an event with Marc at school.
But to go back to your question (sorry for the divagation), I remember at the start, when if I remember well unelss she thinks about incest under the shower, the incest is out, then when she can actually do nothing with him in the laundry and that eliminates further possibilities, then (again, if I remember well), when she spots him watching Anthony's "home" videos, another scene when she goes to him or not, and for sure the pool scene.
In some, it maybe that you only skip that scene but do not close the door for the future, but in at least a couple, I remember unless you chose explicitly to go ahead and do stuff, it would close down the Marc possibility, and in the pool scene, it is even explicitly a decision, communicated even to Marc as such, to stop.
