Why are LBGT studies and queer theory part of philosophy now?
They have become part of philosophy along with an overall interest in expanding cultural studies to include attention to issues previously neglected. This change has been part of the humanities, generally, and philosophers have focused on conceptual issues related to these fields.
Queer theory emerged in the 1990s, along with LGBT (Lesbian Gay Bisexual Transsexual) studies, as a positive affirmation of sexual difference that does not fit into any of its predecessor categories, including lesbianism. Good overviews on the subject may be found in Naomi Schor’s Feminism Meets Queer Theory (1997), and helpful works on transsexuality are Susan Stryker’s Transgender History (2008), and Laurie Shrage’s You’ve Changed (2009).