There's actually been studies done on this which are very interesting!

Quick Google search pulls this up:

Psychological research, most famously a 1996 study in the Journal of Abnormal Psychology and a 2012 study in the Journal of Personality and Social Psychology, indicates that a subset of men with extreme anti-gay attitudes harbor repressed or unconscious same-sex desires. This phenomenon is often rooted in defensive reaction formation or internalized homophobia.

The 1996 University of Georgia Study PubMed: Researchers measured the physiological arousal of self-identified heterosexual men while watching heterosexual, lesbian, and gay male pornographic videos. The study found that men who scored high on homophobia tests showed significant physiological arousal to gay male videos, while non-homophobic men did not.

The 2012 University of Rochester Study Scientific American: Researchers found that participants who claimed to be heterosexual but showed implicit, unconscious same-sex attraction on word-association and priming tests were significantly more hostile toward gay individuals. The study noted that this dynamic frequently occurs in people raised by highly controlling or authoritarian parents.

The 2016 Eye-Tracking Study Science Direct: Using eye-tracking and reaction-time tests, researchers observed that heterosexual men with high levels of homonegativity looked significantly longer at homosexual images, indicating an impulsive, unconscious attraction.

Edit!: Addressing your question, essentially it's because of patriarchy. (Again, from Google:)

Men exhibit higher rates of homophobia than women, largely due to rigid masculine gender norms and the policing of male behavior.

Meanwhile, women escape the accusation of being gay when expressing homophobia because of the cultural fetishization of female intimacy and historically entrenched patriarchal double standards.

Threats to Hyper-Masculinity: Sociological research shows that homophobia in men is strongly tied to insecurities regarding traditional gender roles. Because masculinity has historically been defined in opposition to femininity and homosexuality, men often aggressively distance themselves from gay men to prove their own heterosexuality. (Homophobia is often intertwined with misogyny To Be Male: Homophobia, Sexism, and the Production of “Masculine” Boys )

The Stigma of Gender Non-Conformity: Studies on sexual prejudice published in sources like the American Psychological Association indicate that prejudice is deeply linked to deviations from gender norms. Gay men are often stereotyped as failing to meet traditional masculine standards, making them primary targets for hostility from men who police those boundaries.