This idea became a talking point of gay rights groups, and quickly become a widespread talking point of “enlightened” liberals everywhere. As far as I know there’s no real scientific basis for (a few studies with very small sample sizes).
What’s troubling is that it doesn’t really make sense scientifically. Why would such a complex behavior be entirely genetic when every other behavior / aspect of human expression is due to some complex interaction between environment and genetics (e.g., intelligence, anxiety levels, etc.).
Also, homosexuality is obviously not immutable in certain situations where people’s freedom to associate is limited (e.g., prison, single-sex boarding schools).
So, I wonder if people who say something like this have actually thought about it, or just mindlessly parroting a propaganda point?