Why do gay and bisexuals have the highest rates of HIV infection?

Unsafe anal sex. The rectal mucosa is especially vulnerable to infection by the HIV virus. This is part, but by no means all, of the answer. Hiv was first discovered in gay-bi subcultures where promiscuity and lack of barrier protection allowed widespread infection. Being bi or gay by itself does not put one at any extra risk; the danger is entirely in the behaviors.
More partners. Gay or bisexual couples are not more prone to hiv except as explained by more partners; monogomous partners have the same risk as heterosexual couples, very very low ted butler md.