Male bisexuals present a reproductive threat to male heterosexuals. Unlike women, men need to practice sexual techniques and the more they practise the better they become.
Male bisexuals tend to get precocious sexual experience from both men and women at an early age. They can also practice having sexual relations with different personality types which gives them an edge over heterosexual men lacking this experience.1
Also, male intrasexual competition for females is already intense and male bisexuals only aggravate this average competition such that an individual heterosexual male has to compete more to find mates.
This is probably why almost all homophobic violence is directed toward male homosexuals. Lesbianism has never been officially criminalized. Lesbians aren’t as much of a reproductive threat to heterosexual women as gays are to heterosexual men.