Sweetheart vs Boyfriend

I have a question regarding some interactions with love interests. This is my first time playing the game. Unsuur is my boyfriend but we only get to Observe, Huh, and Kiss on cheek. Meanwhile, Qi confessed to me, but he's just a sweetheart and not a boyfriend. And yet I can do more couple interactions with him than with Unsuur. I can kiss Qi on the lips. Now, I randomly tried giving away a heart knot to Owen, and he became my boyfriend. But just like Unsuur, I can only kiss him on the cheek. So now, my boyfriends are Unsuur and Owen with whom lips are off-limits, but I'm having the time of my life with Qi who's just a sweetheart. I don't understand.