Incest relationships exist in a sort of secret, "underground" world, but the vast majority of consensual incest couples are completely isolated and marginalized by the rest of society. Millions of people are incestuous, but very few talk about it, nobody admits it, and the "ick" factor from society is punitive and harmful. People involved in consensual incestuous relationships roughly occupy the same territory that homosexuals did in the 1950's. "deformed babies" argument no longer holds water, and truthfully, never did. We have intrauterine genetic testing like amniocentesis. We don't criminalize people with a high risk for Tay-Sachs, or cystic fibrosis, or neurofibromatosis, so why is society so obsessed with the genetic risks of incest? We have birth control. Genetic anomalies can result from many things. There are the obvious problems of genetic anomalies if children result from a first-tier incestual relationship, but the risk is only a few percentage points higher than a mother over 40. History is rife with stories of romantic relationships between family members. The very fact that an incest taboo exists at all indicates that it was a common thing in society. Today, we are struggling with gay marriage and all the other civil rights that married heterosexual people enjoy. People who practiced BDSM or other fetishist sexual practices were considered sexual perverts.
For years homosexuality was considered a mental illness.