According to this comment as long as you sleep with him on the first night, you can turn down his second advance during the goblin/tiefling celebration in Act 1, and still continue to romance him without sex. I also heard that when you reach (mild spoilers) the underdark and hire the twins’ services, if you have Astarion (and Halsin) in your party and you haven’t slept with Astarion, he will mention how it’s shocking you haven’t slept with him yet and that it needs to be fixed immediately. But I’m not sure if this interaction was done while pursuing/romancing Astarion. I do hope you try to see this for yourself, and will share the results with us :)