Originally Posted by Spock
That aside, about the incest: Mark and Neverwhere cannot have sex simultaneously with you without there being an incestuous atmosphere. Perhaps you've never considered that as a possibility? I know I'm not interested in my brother's sex life.
. . . though it's unclear what "simultaneously" means here.
It might mean group sex - a specific set of acts in which all three participate together. As noted, that need not be the case in a poly relationship.
Or it might mean that she is a sexual partner to each the two brothers in the same general time period.
To call that incestuous might make sense in terms of traditional patriarchy, especially in keeping with the myth of female purity: the two brothers could be seen as engaging in an incestuous act only on the basis of the notion that one comes into contact with some kind of impurity or residue left behind by the other.
But that's really just bad metaphysics.
There is no such impurity or residue or essence or whatever.