The preposition agains in Slavic languages (and Romanian)

I was thinking the other day that Czech/Slovak proti (against) and Polish przeciwko are followed by the dative case, but Russian protiv is followed by the genitive. Macedonian, having no cases, won't be discussed here. Since those are the only Slavic languages I know, I decided to research how other Slavic languages behave in this aspect, and I've found that Slovenian/Serbian/Croatian protiv takes the genitive, like Russian, as well as Ukrainian navproti. It seems too great a coincidence that all those languages (except Ukrainian) have a preposition ending in the v sound, whereas Czech/Slovak and Polish prepositions end in something else. Another language, albeit not Slavic, came to mind, Romanian, probably the only Romance language that has retained some of the Latin cases. Romanian contra takes the genitive/dative case, which have combined into one in Romanian, not in accordance with Latin, which required the accusative case after the preposition contra. Could it be a Slavic influence? Languages mostly influence one another lexically, so such an influence would be unlikely, but one never knows.