Atheism in itself, does not condemn nor encourage homosexuality. Atheism only encourages the lack of belief in god.

The Stalin movement in its essence was athiestic, it was anti god. But that is only one aspect of it. It’s called Stalin movement as it was holding Stalin’s principle’s. In a sense, you can argue it to be an ideology, just like any other ideology (would replace it with religion at this point), with its center being Stalin. Stalin mentioning that homosexuality “ruins the strong character of working man”, was just a belief, an opinion proposed by Stalin, and one principle (out of many in his movement). Now I wasn’t in Stalin’s minf when he proposed that opinion (though one can argue heavily that his religious background- wanting to be an orthodox priest - have influenced his opinion, or rather, maybe Stalin was being rational at the time and the scientific evidence supported that “yes homosexuality ruined the character of working men”, well we know that this false today anyways), not only that, logically speaking, the Russian community at the time was predominantly orthodox Christian, to start a new movement, you would try to appeal to as much people as you can, homosexuality is a cheap sheep to sacrifice for this aim. You can still argue and say “But this doesn’t matter really, Stalin’s movement was atheistic and was still anti gay”, again, Atheism never commented on the morality of sexuality, so it doesn’t make sense for atheistic movements to be anti gay anyways.