Jellyfish Life, My Neighbors Lonely Wife, and Runaway Girl are my top 3 picks. They are not sandbox but cohabitation type games, you have jobs and collect money and buy stuff and have choices to interact with the girl. They all have one heroine only. The progression is gradual, then first sex, then sex without condom. However, the 3 of them also could of used more variety of sex scenes/gallery CG. But they are all top tier of this genre imo.