Science, technology, engineering, and mathematics (STEM) career pathways can be both lucrative and transformative in regard to social change (e.g., Christensen, Knezek, & Tyler-Wood, 2014; Fuesting, Diekman, & Hudiburgh, 2017; Wang, Ye, & Degol, 2017). However, research shows that STEM learning environments are inhospitable to students with minoritized identities--most notably, women, people of color, and people with disabilities (e.g., Gottfried, Bozick, Rose, & Moore, 2016; O'Brien, Blodorn, Adams, Garcia, & Hammer, 2015; Stout, Grunberg, & Ito, 2016). Almost no research has examined the experiences of students with minoritized identities of sexuality and/or gender (MIoSG) in STEM. Using grounded theory, this study documented how 56 students with MIoSG experienced and navigated a pervasive culture of centering cisgender heterosexual men within STEM learning environments--referred to by students as dude or bro culture. Rich quotes from participants across gender and sexual identity spectra described how dude/bro culture included bonding over hypermasculinity, assuming heterosexuality, perpetuating anti-lesbian, gay, bisexual, transgender, queer, intersex, and asexual discourses, treating students with MIoSG as if they are not smart or invisible, and objectifying and sexualizing cisgender women. Implications for higher education practice are provided.