Outcest is shorthand for outcestosexuality (which is sexual behaviour and
attraction towards people of a different lineage)
The other one is incest, shorthand for incestosexuality (sexual behaviour and attraction towards people of the same lineage).
While current studies only say 98% of the population is outcestosexual, some sociologists believe it could be due to societal repression. Truer estimates in the future could be as low as 58% outcestosexual as society becomes more tolerant with kids exploring their sexuality with siblings, as well as visiting incest bars to explore their sexuality with incestuous adults who roleplay as parental figures (since parenthood is a
social construct).
Unfortunately, incestophobic tropes represent incestuous people as groomers, a bigoted slur.
Rights organizations hope to legalize same lineage marriage and adoption rights for incestuous couples who seek to pleasure their sibling-spouse and adopted kids.