Those who contend that, historically speaking, marriage is a male-female phenomenon only are, in effect, wrong.
In fact, there's good evidence for rites of male-male bonding—a functional equivalent of marriage—across the Indo-European-speaking world.
Such a rite survives to this day among the Kalasha of what is now northwestern Pakistan, the only IE-speaking people who have practiced their traditional religion continuously since antiquity.
The Goat at the Heart
Being a mountain culture, the Kalasha are aigocentric: goat-centered.
Like the Celts of ancient Britain, Kalasha culture is transhumant. During the Summer, the young men take the flocks of goats up to the Summer pastures in the mountains and live there together for months at a time.
It's unsurprising that intense emotional relationships should spring up between these young men. When two of them wish to make a lifelong commitment to one another, it's time to enact the traditional rite of, in effect, blood brotherhood.
An Act of Mutual Adoption
Together, the two sacrifice a goat to Sájigor, the protector of flocks. (Here in the West, the Horned has always been patron of male-male bonding.)
Having slaughtered the goat, they roast its kidneys over the fire. They then feed one another pieces of the kidneys on the tips of their knives.
Then they suck each other's nipples.
Though pungent with symbolism throughout, it is this final act which articulates the rite's central meaning. Across the Indo-European world, the act of suckling figures as part of the rite of adoption.
The Kalasha rite of blood-brotherhood constitutes, in effect, an act of mutual adoption.
A Pan-Indo-European Phenomenon?
Nineteenth century travelers' accounts make it clear that this rite was once common among the cultural kin of the Kalasha, the so-called “Kafiri” cultures of northwestern Afghanistan, now—since its forcible conversion to Islam during the 1890s—called Nuristan, “land of light”.
In fact, British consul George Scott Robertson undertook the rite with Waigali warrior Shermalik, and wrote of it in his 1896 book The Kafirs of the Hindu Kush, though he clearly didn't understand the implications of what he was doing.
We may suspect that similar, parallel rites of male-male bonding once occurred across the Indo-European-speaking world. As among the Kalasha, traditional societies tend to be structured along lines of kinship; such rites serve to build ties between kinship groups, and are hence indispensable for long-term cultural stability.