As a bisexual guy, I found this topic particularly intriguing!
So I've compiled a research-based overview of how lesbians, gays, and bisexuals perceive muscularity.
1. Some Prefer Thinner Body Types
Some gay and bisexual men might link muscularity with heteronormativity, conformity, or superficiality, leading them to prefer thinner body types and alternative expressions of masculinity. 
Another article just on this topic:
Why do (some) girls like skinny guys?
Check out the full article...
2. The superficial desire for muscularity
Conversely, certain gay men might seek muscular and athletic partners to compensate for a perceived lack of masculinity (in the heteronormative sense) or to avoid bullying or rejection from other men. 
3. Self-esteem effects attraction to muscular men
4. Attraction to lower body fat levels
Similar to heterosexual women, gay men find muscular men with lower body fat more attractive as short-term partners, but less appealing for long-term relationships. 
Click the thumbnail to read my full rant article about why muscles are attractive and how it changes throughout cultures!
5. Gay men prefer muscular men
Generally, gay men prefer men with greater muscularity and lower body fat than women do. 
6. Lesbians Prefer Partners With Lower Body Fat
Research indicates that, while lesbians' preferences for muscularity do not differ from straight women's, they tend to prefer partners with lower body fat. 
If that all seems like a complicated mess of words, here's a nice infographic showing how lesbians, gays, and bisexuals perceive muscularity:
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 Cordes, M., Vocks, S. & Hartmann, A.S. Appearance-Related Partner Preferences and Body Image in a German Sample of Homosexual and Heterosexual Women and Men. Arch Sex Behav 50, 3575–3586 (2021). https://doi.org/10.1007/s10508-021-02087-5
 Lanzieri N, Hildebrandt T. Using hegemonic masculinity to explain gay male attraction to muscular and athletic men. J Homosex. 2011;58(2):275-93. doi: 10.1080/00918369.2011.540184. PMID: 21294030.
 Eleanna Varangis, Nicholas Lanzieri, Tom Hildebrandt, Matthew Feldman,
Gay male attraction toward muscular men: Does mating context matter? Body Image, Volume 9, Issue 2, 2012, Pages 270-278, ISSN 1740-1445, https://doi.org/10.1016/j.bodyim.2012.01.003.