I don’t think that’s the argument anyone is making. If they are, it’s ineffective.
Atheists and science-centric individuals point out such things as “gay penguins” because those on the far-right constantly assert homosexuality is unnatural and a choice. Classifying homosexuality as such allows them to demonize it.
No one is trying to imply homosexuality gains legitimacy from its prevalence in other species. What’s being illustrated is that homosexuality is not the perversion or anomaly some paint it as.
Even if it was an anomaly in humans, I’m pretty sure atheists would still be in favor of LGBT rights since most atheists are in favor of consenting individuals living their lives how they wish—just as long as they’re not forcing their personal ideology on others via laws, government, etc.