In many cases the manga / manhwa (or anime / light novel, which you did not mention but is the same concept) itself is already functioning as an established IP. The otome game is often made to capitalise on an existing fanbase.
Whether it "spoils" the game probably depends on how closely the adaptation follows the source material. If the routes, twists, and character dynamics are mostly identical, and only one guy is still the "main love interest", then people who already consumed the source may feel less incentive to play because a large part of the mystery or emotional tension is already known.
At the same time, the otome game adaptation can still offer something different even if the world setting is familiar, for instance, with new routes that breaks away from the canon story where other male characters get a chance with the heroine. Or in other cases, by introducing a totally new heroine that never did exist in the source material who has her own dynamics with the characters.
Depending on the execution, familiarity with the source may actually increase enjoyment rather than reduce it. At any rate, there is diversity in adaptation quality so there is no one fit answer.
Personally, I don't think prior exposure automatically ruins the game, but it does lower expectations for me. If the adaptation does not expand enough beyond the original material, then it can end up feeling more like fanservice for existing fans than a standalone experience. And in other cases, it sinks my ships so I'd like to pretend the adaptation's route is just fanfiction (e.g. I shipped Senri & Rima from Vampire Knight, and they're a canon pair, but Senri is still a love interest in the DS game).