Because both aspects are essential to the definition of marriage.
Entities which are not fully “other” to each other, intrinsically, cannot, by definition, unite.
It has nothing to do with “being an outsider.” It has everything to do with language, and definitions which depend on meaning for their validity. And how would you, an outsider to me, possibly know that I was not referring to “love felt between two people?”

That’s not part of the discussion. Marriage is not about (subjective) feelings, but about (objective) realities. A man and a woman in an imperfect marital relationship are truly married, whatever the quality of love is between the two.
Nope. What distinguishes marriage from non-marriage is the radical category, one from the other. Thus, a heterosexual couple who is fornicating does not transform their relationship into “marriage” by declaring the quality of their love to be sufficient or equal to married love. They are, radically, unmarried.
What differentiates one relationship from another is the quality of that relationship. But a subjectively “higher” quality relationship between any two people (vs. their subjective perception of what any other two people posses) does not a marriage make.
“Outside” and “inside” refer to subjective perception. Marriage is defined not by subjective perception but by objective criteria. Don’t have the essentials? You’re not married. All the legislation in the world will not make you married if you do not possess those essentials. It will allow you to pretend at marriage.