You can pick and choose who you romance. I think it just enables/disables the various routes. The romance paths kind of occur in parallel to each other and culminate either into monogamy or a harem but the game isn't far enough along for that yet.
Most romance routes involve the main character being nice or cordial to the ladies I think a fair amount of them actually initiate the sex themselves and the protagonist just goes along with it. You have a few characters like the prostitute character who you can either take possession of and pimp out, or free her and let her start a business.
