That's why Poseidon rejected Kotsume cause Kotsume is more comfortable and treated well in the underworld. If Poseidon accepted him, it's possible that he will not happy there considering how he received harsh treatment from his colleagues.
I'm really heartbroken seeing Kotsume crying but...
If he really love her, he should respect her feelings. Not forcing her and even imprisoned her. I feel like he is playing with doll or a pet.
I don't know why author thinks this kind of abuse is okay if you just fall in love each other later.........that's really an unhealthy relationship...