I hope I've gotten the history right. My understanding is that the Bolsheviks repealed the entire Tsarist legal code, which included prohibitions against (male) homosexuality, and didn't make any laws on it one way or the other. This made homosexual relationships technically legal, but also, given the general conservatism of Russian culture at the time, put them in a kind of strange gray area, since many officials persisted in considering homosexuality bourgeois decadence and identical with pederasty. Then the government started regulating homosexuality much more strictly, as part of an effort to stamp out "bath-houses" and other illicit sexual entertainment venues that were causing a spread of venereal disease. Under Stalin, who got several things wrong despite being one of the greatest men in modern history, the ban on homosexuality was made explicit. It seems that Soviet psychology in the 1930s, being like all psychology of the time influenced by Freud, believed that fascism and homosexuality were heavily linked -- i.e., if you're that into guns and tight uniforms and virility and the inferiority of women, there's clearly something phallic going on.

So it's a mixture of bad science and trying to fix, without really knowing how, certain genuine social and medical problems. Along with homosexuality, abortion was made illegal, and divorce became harder to obtain. Kollontai, interestingly, actually campaigned for some of these "conservative" changes -- not criminalization of homosexuality, but certainly banning abortion and tightening divorce laws. Soviet women's organizations were behind them as well. Apparently there was a perception, I don't know how justified, that some women were taking advantage of the loose divorce laws that existed in the early Soviet Union to "poach" mates, thus undermining the cohesion of local communes, and engaging in a type of social parasitism; if today, in our fairly liberal society, there exists a strata of conservative women who think that all other heterosexual woman are just temporarily embarrassed homewreckers, the perception can only have been stronger in a country just emerging from feudalism.