Here's a non-scholarly source to get up to speed on the hype.

Boswell argues that adelphopoiesis was used to recognize same-sex relationships. Given the Roman historical context of the early Church, what of the ideology of the early Church we have access to, and several scholars seriously questioning Boswell's translations, Boswell's position is definitely fringe and somewhat unlikely.

You can read Boswell himself for the argument, but any sort of database search for criticism, and you'll find it, in quantity and quality.

Well, there's the claim that the Victorians "invented" homosexuality, because they were supposedly the first to identify homosexuality as a cultural identity. Foucault talks a lot about the Victorians in his History of Sexuality, and is especially interested in why we are so invested--as Ian McKellan seems to be--in the myth of the Victorians being very repressed about sex. (They really weren't.) Oscar Wilde's trial is also seen as a turning point in the history of homosexuality, and some claim it's one of the first times sexual acts were solidified into sexual identity. The idea is that men have been having sex with men for centuries, but it was only after the late 19th century that they would be put into the social category of "homosexuals."

For an alternate perspective, Sharon Marcus's Between Women argues that female sexuality in the nineteenth century was much less regulated, and what we might now call lesbian affection between women was a natural part of many female friendships. Many women even lived together in arrangements similar to legal marriages between same-sex partners today.

Krafft-Ebing's work was also very influential, and from him we got the idea of what he called "sexual inversion"--for example, that lesbians are "men trapped in women's bodies."