I'm not sure what qualifies as a porn game to you but story based novel is a good description for the game. There are indeed less erotic scenes at the beginning because you have to get to know your LIs first. The scenes become more intimate/kinky as your relationships and the story progress.
As for the pacing, let me be blunt with you: This is not a game you'll pull up when you want to masturbate. There's a gallery for unlocked erotic scenes, but the game generally prioritizes story and then throws occasional erotic scenes into the blend. Since those scenes are rare and have a lot of setup behind them, they feel more special and rewarding.
I'd recommend abstaining from porn for a few days before playing this game since you'd otherwise be too numb to notice the nouance put into it. When you're playing Eternum, you're in for the long haul. The story has a way of distracting you from why you started playing the game in the first place.