If memory serves, brother-making has parallels with the concept of "blood-brotherhood". It was a way of sacramentalizing close friendships and ending disputes between political/social rivals, and strengthened interfamily/clan/tribal alliances. It created a relationship within the Church that was sufficient to bar marriages between members of the natural families of the "cross-brothers/sisters" joined in the brother making.
It is an ancient ceremony which is mentioned in the Rudder.
That said, it fell out of use in many places in relatively recent times precisely because it was being used too often as a cover for other types of otherwise unblessed relationships. It was…and from time to time, still is abused in this way. Thus it has become rare and effectively extinct in several jurisdictions…one of those things, still permissible, but not really done much anymore.
Prohibitions: Brothermaking, as well as being a God parent was forbidden to monastics.
Anachronistic readings/interpretations of brother making.
Consider the ideation associated with the terms "making love, lovers" most people hear it today and instantly conceive of a sexual encounter. But all it takes is a little late night TV to show how the term has been debased in the last generation or two. Look at old episodes of I Love Lucy and from time to time you will hear Ethyl speak of her and Fred "making love". We think sex…she meant sweet nothings exchanged on a park bench in the moonlight. Big difference.
Kissing…well up until relatively modern times kissing was a greeting, a near universal Christian greeting…one we still see in Europe and in the Church. Kisses on the lips were gestures of affection regardless of gender. Judas kissed Jesus…even as late as Shakespeare's time two men kissing in Merry Old England was just the greeting of friends…not that of potential sexual partners.
Sleeping in the same bed: Again up until relatively recently sharing a big bed with several others, even in an inn was pretty common. There was nothing sexual about it.
Sts. Sergius and Bacchus: It is worth noting that they were very close friends even before they became Christian. It was close enough, and intimate enough that the Romans in their efforts to humiliate them dressed and paraded them in women's clothing on their way to their martyrdom. This, however, need not have any scandals implication…such humiliations of Christian soldiers was practiced by Julian the Apostate. The nature of their martyrdom has unfortunately fed the "romantic" speculations and applications of their lives to the subject of gay marriage.
So its easy to see that there has been a lot of casual misreading of the accounts of these ceremonies, perhaps out of ignorance…perhaps out of an agenda.