I don't think that marriage would ever be a proper form for the same-sex partnership
Here I think that Slavs needs to relax. Marriage itself as it is, between man and woman, is institution in question. So, no harm if marriage for the same-sex become legal. Key word here is `law`. In other words, to legally define things between partners, for example- question of property if some of them dies. What I want to tell you, if Slavic societies tolerated all possible intimate differences and practices for eons already, no need to defend institution such is marriage, that is anyway, as institution, novelty and result of new era.
Now, what do you want to say by saying "the beginning"?
Who knows what future can bring to us. Defining marriage among clones or between clone and non-clone. Maybe even extreme suggestions for marriage between humans and animals, to put cross on myself. Who knows
against the adoption of children
Absolutely. This should be bottom line in Slavic societies. Child can be adopted only by couple that consists from different sex partners. Why? Logically and simple, because child coming as result of way of life that follows different sex partners. See? Very simple. It even isn`t for discussion when you put things logically. No need to fall in western European trap about `human rights`. Logic suggest that child have right to follow patern of life created by its parents. Later, when child grow up and become person; person can decide about some new way of life.