To justify a ban on homosexual marriage you have to come up with a reason why they shouldn't marry each other. You're not really addressing that at all.
For one, it's internal politics. Russia has a record of having a "strive after the bright future" master narrative for over 70 years. With the fall of the USSR that narrative fell with it, and there was no master narrative at all but the wild money hunt for a decade. Then many people got tired of that, and started searching for some supreme idea. Because most people are not creative enough to invent something brand new, they searched in the past, ransacking dusty old ideas, and eventually dug out traditionalism and religion to cling to. Neither gets well along with homosexuality, but both are currently on the rise. So it's a popular decision, that's all.
For two, there is no particular reason why homosexual marriages must be supported as they are not demographically productive. And demographical production is a problem, because there was a gap in birthrate in the 1990s due to considerable drop in the quality of life multiplied with total economic insecurity. Now that starts to echo in lack in young adults of reproductive potential as kids of the 1990s come of age, which can result in another gap in the birthrate.
It is known that "true" gays are gays and there's nothing on Earth that can make them straight. But efforts are made to reduce potential number of youth to try homosexuality out of curiosity and then to make a habit of it although they are able to conduct heterosexual lifestyle.
The combination of the two result in the... well, I wouldn't call it a ban, because to ban something you must have it for a while first - like public smoking, which was first allowed and then banned (or may be I just don't understand the word "ban" right).
Homosexual marriage was never allowed in Russia, and there seems to be no reason to change that now.
So First) a prohibition based on children and what age is appropriate to give consent, I can roll with you here. 16 might be a little young, but it's not obviously terrible. So that's ok.
Praise the lord! At least something
Second) a wrongness
Third) a wrongness
Fourth) a wrongness
Second and Third - brings up social tensions (as monogamy fits the traditional and religious paradigms mentioned above) and increases risk of social vulnerability in case something happens with the harem lord (or mistress).
Fourth - involves unacceptable risk of genetic abnormalities in the offspring.