People often don’t like when their significant other says they belong to each other. I think it depends on the relationship. In a good relationship, calling your partner ‘yours’ is a term of endearment. In my mind anyways. I think this stands true if you’re in polyamorous/open relationship as well.
From Protector of the Grey House:
“If you hadn’t stopped me in time…”
He inhaled sharply at the devastated look in her eyes. His eyes narrowed a touch as he caressed her cheek. He had never seen her this distraught. Vincent’s own expression grew soft as he drew her into his arms. He kissed his woman and held her tightly to him. She wrapped her arms around his neck and returned the kiss. After a moment he pulled away and whispered into her ear.
“Tell me you’re still mine.”
“Tell me I’m still yours.”
He had never asked her that before. Vincent often referred to her as being ‘his’, but never questioned that she wanted him. Natalia wanted to draw back to look into his eyes, but he wouldn’t let her. His arms tightened around her and held her still. Natalia moved closer still and spoke softly. “Yes, you’re still mine.”