I interpret Sesshomaru as more of a father figure to Rin so I personally dislike them in a relationship as I think there is something so much purer and sweeter about a demon who hates humans fathering a human child and learning selflessness through that love as opposed to Sesshomaru meeting a human girl, protecting her for most of her childhood and then becoming her husband.

Sesshomaru being her father just personally felt more organic and natural as a progression.

Especially given the fact that Sesshomaru does in many ways act as Kohaku's guardian later in the series.

I like to see at as him making up for failing Inuyasha as a child and changing to not make the same mistakes to the children in his care now.