I feel like Boswell's work is...um. Complicated. (I'll leave it at that because homophobia absolutely affected the reception of his work at the time, but I also think that he's not really cited UNCRITICALLY now.)
Clare A. Lees (ed.), Medieval Masculinities: Regarding Men in the Middle Ages (1994)
Louise O. Aranye Fradenburg and Carla Freccero (eds.), Premodern Sexualities (1996).
Karma Lochrie, Peggy McCracken, James Schultz, Constructing Medieval Sexuality (1997)
Mark Jordan, The Invention of Sodomy in Early Christian Theology (1997)
Carolyn Dinshaw, Getting Medieval: Sexualities and Communities, Pre-and Postmodern (1999)
A seminal work is Glenn Burger and Steven Kruger (eds.) Queering the Middle Ages (2001). Seriously, this is one of the top books I'd recommend OVERALL if you want to get the hang of it.
Francesca Canade Sautman and Pamela Sheingorn, Same Sex Love and Desire Among Women in the Middle Ages (2001)
Sharon A. Farmer and Carol Pasternack (eds.), Gender and Difference in the Middle Ages (2003)
Glenn Burger, Chaucer's Queer Nation (2003)
Richard E. Zeikowitz, Homo-Eroticism and Chivalry: Discourses of Same Sex Desire in the Fourteenth Century (2003)
William Burgwinkle, Sodomy, Masculinity, and Law in Medieval Literature : France and England , 1050-1230 (2004)
Tison Pugh, Queering Medieval Genres (2004)
Karma Lochrie, Heterosyncracies: Female Sexuality When Normal Wasn't (2005)
N. Guynn, Allegory and Sexual Ethics in the High Middle Ages (2007)
Louise Sylvester, Medieval Romance and the Construction of Heterosexuality (2007)
David Clark, Between Medieval Men:Â Male Friendship and Desire in Early Medieval English Literature (2009)
Tison Pugh, Sexuality and its Queer Discontents in Middle English Literature (2008)
Ruth Evans, A Cultural History of Sexuality in the Middle Ages (2011)
Joan Cadden, Nothing Natural Is Shameful:Â Sodomy and Science in Late Medieval Europe (2013)
Larissa Tracy, Castration and Culture in the Middle Ages (2013)
Anna Klosowska Roberts, Queer Love in the Middle Ages (2016)
Victoria Blud (ed.), The Unspeakable, Gender and Sexuality in Medieval Literature 1000-1400 (2017)
Kent L. Brintnall, Joseph A. Marchal, and Stephen D. Moore (eds.), Sexual Disorientations: Queer Temporalities, Affects, Theologies (2017) (NOTE: I cannot help you with anything related to this because, to be perfectly frank, queer temporality is one of my LEAST favorite branches of queer studies. It is valid and the scholars who study it are valid but I Don't Like It.) (And because the people I've known who've studied it have been cishet twats.)
Ann Marie Rasmussen, Rivalrous Masculinities:Â New Directions in Medieval Gender Studies (2019)
Will Rogers and Christopher Roman (eds.) Medieval Futurity: Essays for the Future of a Queer Medieval Studies (2020)
Marina Montesano, Cross-dressing in the Middle Ages (2024) (NOTE: I get why this title alone might make people nervous, but my understanding of it is less that it's equating trans identity to cross-dressing and more that she's covering the various ways and contexts cross-dressing could be utilized in the middle ages, inclusive of people who would possibly be trans today.)
Jacqueline Murrary, From Texts to Bodies:Â Sexes, Genders, and Sexualities in Premodern Europe (2025)
This seems like a lot and it is a lot, and that's not taking into account the journal articles. My advice is to look at the table of contents, read the introductions, and figure out what interests/is most applicable to your interests. Pay attention to how they speak about the field and speak about one another and their work, because queer studies has changed a LOT since the early 90s, and, even at the time, scholars will often disagree on things.
Also, since you're an undergrad, I recommend getting your theory reading out of the way, since you're going to need it. Read up on Foucault but ESPECIALLY Butler, Gender Troubles is your friend when people start talking about how "can we REALLLLLLLYYYYYYYYYY say that they were queer back then?" Or "But HOW are they queer?" Develop the vocabulary to talk about these things, it'll save your neck in the long run, especially when you're being cornered by your cishet colleagues and need to think on your feet. I also like Lee Edelman's work on queer futurity, since it's applicable to a lot of medieval Guysâ˘, though it's a little LESS necessary. When you're writing a paper and ESPECIALLY when you're writing a thesis, make sure to establish your theoretical framework.
Because of the huge overlap between monster theory and queer theory, I highly recommend at least reading Jeffrey Jerome Cohen's "Seven Monster Theses" in Monster Theory. "What does Hannibal Lecter have to do with Grendel and The Green Knight"? YOU'LL FIND OUT. (...that sounded more ominous than I intended.) This is also because I think everyone should read Jeffrey Jerome Cohen's Monster Theses at least once. Just never listen to anything he says about the Welsh Marches.
Also see: Will Stockton, An Introduction to Queer Literary Studies: Reading Queerly.
From this point, you can go into the individual sub-areas of the Middle Ages you want to go into. It's good to keep a library that covers multiple areas, since it means you can draw from them to prove a point in a related field that might not have been considered before. (As you go into grad school, careful cross-comparisons are your FRIEND, take advantage of other scholars' tendencies to stay in their own little bubbles to astound them with the research that's being done elsewhere.) Beowulf, for example, has a different reception history re: queer studies than Sir Gawain and the Green Knight, which has a different reception history than the sagas of the Icelanders, which has a different reception history to the TĂĄin BĂł Cuailgne. Arthuriana is essentially its own super-field that has its own little nuances in general.