[ahje]

The LGBTQ-movement, for example, is extremely vocal about any technique that could potentially be used to determine the sexual orientation of a person. They've spent a huge amount of time and resources in order to get society to redefine homosexuality from an illness and into a personal matter which should be included in societal norms.

If someone discovered a reliable way to detect the future sexuality of a child then those efforts would suddenly be contradicted, because 1) it would be harder to argue that it's not just a condition, and 2) no matter how controversial, there would be a demand for such screenings, and the LGBTQ-community would most likely interpret it as an existential threat.

Obviously, I chose sexuality as an example as it's an easy metaphor that most people get, but this reasoning can be applied to quite a few other things that are (possibly) caused by genetical factors and where there's a community involved. As an example, I've heard people mention an on-going "genocide" of people with Downs syndrome, simply because pre-natal screenings are quite effective at detecting it and that parents therefore terminate the pregnancy instead of having a child with Downs.

EDIT: Simply put: If you start screening for X, thereby allowing it to be prevented or treated in any way, then you imply that the X is something negative. People with X might take offence to that.