I think it's the idea that a person's sexual identity, hir sexual being, can be reduced to the same level as "action-adventure" or "romance." It feels dismissive. Maybe somebody else should chime in. *looks around*

It doesn't feel dismissive to me, as a lesbian. I think "gay and lesbian literature" is a perfectly valid genre, and one I look for in my book store and library all the time. And there are plenty of sub-genres within it - lesbian mystery, gay erotica, coming out stories, etc.