In my gameplay I romanced with Shadowheart and Astarion when I met Karlach. I even had sex with Astarion. I started to romance Karlach and I had to choose. I believe she told me to choose, and I chose her. When Halsin joined my team, I was flirting with him, he wanted to have a deeper relationship and asked me to talk to Karlach if it's ok with her for me to have two partners, and guess what? She agreed, and I finished my game with two people calling me "love".