You might not recognize the queer artist from ''Andy Warhol Retrospective,'' a majestically installed career survey on view this summer at the Museum of Contemporary Art in Los Angeles. Warhol didn't care, or pretended not to, and just by being himself, a public sissy, he automatically became one of the important political artists of his time. This was in the 1950's and early 60's, before Stonewall and gay liberation, when to do so meant to be shunned by many of his artist colleagues, gay and straight. Warhol was also the first major postwar artist to put gay identity - or queer identity, to use the term now favored by many gay men and lesbians as an ironic badge of pride - at the very center of his work. Gerhard Richter, among many other artists, would not exist without him. He made it look trashy and valuable, passive and active, like nothing and like something. He helped change our idea of what art is and what it can do. ANDY WARHOL was the most important American artist of the second half of the 20th century.