Boswell is almost certainly not the best source for this. Eastern Christianity was as negative as Western, and in the Empire at least its canon law was enforced by a somewhat vicious state apparatus (a lot laxer in Russia where canon law and state law barely intersected).
Further, the Church in late medieval Russia really disliked brother-making. It really complicated their idealised top-down relationship model where other members are auxiliary to the father who in turn in subject to a social superior, and of course had impact on various property laws. Because of high-risk agriculture and the relatively traditional nature of the area though extended families and claimed kinships stuck around for a long time. So I think in the general sense there may have been same-sex romantic relationships in the whole affair somewhere but it's almost certain they would have been disguised.
Boswell's only important in that he brought he subject up, his analysis is pretty weak and somewhat Western-centric, leading to lots of hopeful misunderstandings. It's hard not to sympathise, since being deliberately written out of history for a long time really does suck, but yeah. Wouldn't rely on that book too much.
In the Eastern Roman Empire the lines between Church and State where rather absent (in the rest they were very intertwined, but were still distinct entities) and so were a lot harsher. Didn't the Church become subject to the state with the assumption of the title of Czar by Ivan?
I did not know that about Russia, I have been studying the West and Russia is well, Russia. I have always thoroughly enjoyed the Occident's tremendous capacity for anachronistic thought, for example Boswell's introduction of homosexuality into the Middle Ages. There of course have always been men who have copulated with men and women who have masturbated women, but the concept of homosexuality and heterosexuality as identities simply did not exist. Marriages (among the aristocracy at least) were not based on love, they were based on property and bloodlines and so they simply would not understand the concept of marrying (well the sort of would, but they'd find it absurd). It would be as useful as bull on tits (correct phrase?) and so simply wouldn't happen.
Now to the good part. Boswell in a way was both wrong and right, wrong in that there was no gay "marriage", but right in that there was indeed a thing for two males (or two females) to pledge their love for each other. The major problem is that love back then was a lot more complex thing then it is now, for a book covering love then well I recommend C.S. Lewis's The Four Loves especially the part on Philia, yes it is a bit Christian apologetic, but Christianity had a profound influence during the Middle Ages. For a look at the institution if you will see here or for further reading read The Friend by Alan Bray (note, while the subject is interesting he died before the book could be tidied up and this is a bit of a slog). One is also in a bind about how to extricate the homoerotic from the norm, for example back then it was not uncommon for two males to share a bed, the bed was a place of intimate conversation in addition to sleeping hence the queer English phrase "strange bedfellows". Men wrote familiar letters to each other and some rather esemble love letters, men simply were permitted to express a far greater level of emotion especially to each other. The problem is that people today look back and see two men expressing their love to each other and so they apply what they know about the world which is that love is expressed sexually and this leads to improper conclusions.
I am very sorry that my writing is not very coherent, please forgive me.