There is an easy answer to that. We share a love that just makes us want to be answerable for each other and to spend all our lives together, eg. cooking together, doing groceries & chores together, attending mass & other church community activities together, praying together, being beside each other as we lay down to sleep/pillow talk, etc. And we cannot do some of that without the Sacrament of Marriage.
I do not believe an union between two persons have to be about sex. I used to think that way in my youth but now I am actually quite surprised by the kind of nurturing love that can be shared between two persons who did not put sex as a motivation. Love can be shown by other ways that lasts longer than an orgasm. 
I don't believe it has to be "about sex" either, however, isn't it something that the Church considers an essential part to a valid marriage? To be clear, I want to keep this conversation PG (please) and I do not mean to be disrespectful to you, but maybe others can chime in on this.
Having said all of that, from what I'm reading, I suspect that you are already married. The reason why I brought the attraction issue up is because, assuming you are still married and cannot re-marry, then it sounds like it would still be possible (not exactly what you both WANT, but possible) for you to have this relationship with this person as a great friend without cohabitating. The fact that there is no physical attraction is a huge plus in this regard.