ehm.... i just tried find something, similar to gameplay of LonaRpg. I know- Lona not have analogs, so it's just useless...
Lona is indeed one-of-a-kind with no true equal!
(bootleg fumo courtesy of Lona's VK community)
That said there are other games that have some things in common. Depends what's your priority.
- Female protagonist + pregnancy mechanics? (but otherwise rather shallow) VH and VP were already mentioned
- Dark fantasy with steep difficulty curve and overtones of sexual violence? (but turn based combat, and not that much focus on sex) Fear & Hunger
- Dark fantasy with distinctive art style, fairly open world, and some seriously messed up content you can uncover/commit if you're willing to dig deep? (but male protagonist and turn based combat) Black Souls
- Female protagonist + real time combat + lewd and gruesome bad ends? (but it's a 3D platformer) Holy Knight Ricca
- Just horny open world? Skyrim's tormented body struggling under the combined weight of all LoversLab mods you manage to graft on creating something even Godrick would call a step too far.