Felony by definition means it's punishable by at least one year in prison.
So specifically, why are we incarcerating consenting adults for having sex?
From a moral point, don't do that. From a legal point, stay out of the bedroom.
I'll also add context of this is a very Western belief. Natives of America prevented inbreeding by not marrying within the clan. Your first cousins could be in a different clan and therefore open for marriage.