Procopius on Slavs;

And they live a hard life, giving no heed to bodily comforts

Happy with the answer? ;D

Ok now really, hard to tell. I am not even sure if we can make any concrete data on it. I can only speculate. And my guess is they were far more moderate than Romans & Greeks.  :D But then again my assumption is speculated on Roman accounts of the Germanic people. Not that i wanna equate Slavs with Germanics but since the folk north of Roman lands were culturaly relatively similar in many respects i can imagine they had similar ideas about sexuality. But again perhaps i am wrong. For Germans we get this;

18. For all that, marriage in Germany is austere, and there is no feature in their morality that deserves higher praise. They are almost unique among barbarians in being satisfied with one wife each. The exceptions, which are exceedingly rare, are of men who receive offers of many wives because of their rank; there is no question of sexual passion. The dowry is brought by husband to wife, not by wife to husband. Parents and kinsmen attend and approve of the gifts, gifts not chosen to please a woman's whim or gaily deck a young bride, but oxen, horse with reins, shield, spear and sword. For such gifts a man gets his wife, and she in her turn brings some present of arms to her husband. In this interchange of gifts they recognize the supreme bond, the holy mysteries, the presiding deities of marriage. A woman must not imagine herself free to neglect the manly virtues or immune from the hazards of war. That is why she is reminded, in the very ceremonies which bless her marriage at its outset, that she is coming to share a man's toils and dangers, that she is to be his partner in all his sufferings and adventures, whether in peace or war. That is the meaning of the team of oxen, of the horse ready for its rider, of the gift of arms. On these terms she must live her life and bear her children. She is receiving something that she must hand over unspoilt and treasured to her children, for her son's wives to receive in their turn and pass on to the grandchildren.

19. Thus it is that the German women live in a chastity that is impregnable, uncorrupted by the temptations of public shows or the excitements of banquets. Clandestine love-letters are unknown to men and women alike. Adultery in that populous nation is rare in the extreme, and punishment is summary and left to the husband. He shaves off his wife's hair, strips her in the presence of kinsmen, thrusts her from his house and flogs her through the whole village. They have, in fact, no mercy on a woman who prostitutes her chastity. Neither beauty, youth nor wealth can find the sinner a husband. No one in Germany finds vice amusing, or calls it `up-to-date' to debauch and be debauched. It is still better with those states in which only virgins marry, and the hopes and prayers of a wife are settled once and for all. They take one husband, like the one body or life that they possess. No thought or desire must stray beyond him. They must not love the husband so much as the married state. To restrict the number of children or to put to death any born after the heir is considered criminal. Good morality is more effective in Germany than good laws in some places that we know.

I could be quite wrong thow. Maybe they were like animals in this. At any rate one thing is for certain is that they expanded thru Europe relatively quikly for those times. :P For sure higher classes could afford more wifes. And in the case of King Samo this is for sure.