I have the right to keep and bear arms.
I don’t have the right to force someone to manufacture them or to sell them.
If push comes to shove, then I make them myself, even if a club or a rock.
Additionally, I don’t have to contract with either the manufacturer or the seller to be with me the rest of my life using those guns with me.
Therefore, marriage is uniquely the involvement of 2 people where buying an item (any item) from a seller is not.
The fact that marriage requires the consent of both parties is analogous to the requirement that for someone to buy a gun, someone else must sell it. Someone who cannot find someone willing to sell, trade, or give a gun to him, and who lacks the skill to make one himself, has no fundamental right to "have" a gun unless or until such time as he can either make one himself or find a willing supplier.
There are some significant ways in which marriage is different, but they actually relate to the fact that it's not just about two people's relationship. Many institutions, companies, and individuals engage in a wide variety of generalized reciprocity with married couples; they assume that providing various benefits to such couples will promote the widespread existence of stably-married couples, which will in turn improve society in a fashion that will end up benefiting those providing the benefits). A marriage is not only relevant to the husband and wife, but also to anyone who would consider providing benefits to them on the basis of that marriage.