I can't speak from anything other than my own personal logic and experience, but here's my theory.
The social conservatives are as anti-gay as they possibly can be, and they logically ought to be out there claiming gay and lesbian romance fiction is a serious threat to . . . . civilization. But though they won't admit it publicly, I believe they're more concerned about the real threat of traditional -- meaning one man, one woman, Happily Ever After -- romance.
Traditional romances celebrate and inculcate the conservative norm of one man, one woman, HEA. That's a good thing, in the eyes of the social conservatives. And if that's all romance novels did, the social conservatives wouldn't have a problem with them. They'd be quite happy if women read Grace Livingston Hill, Catherine Cookson, and Emilie Loring from now until Armageddon.
Is this a contradiction? Yes, and no. As I wrote at ADAN Divas:
Most writers and most readers, in my personal opinion, fail to recognize that just because the heroine gets to have great sex does NOT make the story pro-woman. There are contradictions that are rarely explored.
What happened in 1972 with the publication of Kathleen E. Woodiwiss's The Flame and the Flower is that female sexual satisfaction as an expectation was thrown onto the table. It's not just that the heroine has great sex; it's that her pleasure becomes obligatory and the hero has a responsibility to oblige her. His sexual satisfaction -- and her duty to provide for it -- was always taken for granted; her sexual satisfaction was often denied, ridiculed, condemned. But because Woodiwiss published at a time when artificial birth control relieved women of one of the major physical, social, and economic consequences of sex, her heroine's sexuality became part and parcel of her being. Woman's sexuality was turned from The Original Sin into A Damned Fine and Beautiful Thing. The thing that made Eve the fall guy for all of human ills -- her being a sexual being -- got turned around 180 degrees.
Many of the novels that have challenged the patriarchal status quo -- white men on top, women and all others below -- have dealt with female sexuality. Lady Chatterley's Lover. Forever Amber. Peyton Place. Fear of Flying. They addressed the right of woman to own her own sexual pleasure. And if a woman takes back her pleasure, she takes back the power of a man to own her.
Gay and lesbian romances don't challenge that power structure. There is no male-as-owner-of-the-female power structure in the individual relationships. And as long as the social conservatives don't see gay or lesbian romances as interfering with the power structure, they aren't going to bother beyond the general and all-encompassing condemnation of anything that even hints as GLBTQ people as real.
Because if they were to attack the m/m or f/f romance novels, they would have to do so from a position of accepting that gays and lesbians can even experience "romance." That they are people who can love and commit and care, and that, too, is outside the sphere of experience for many social conservatives. (It should also be noted that many men who publicly claim to be social conservatives are also heavy consumers of print pornography which contains more than its share of f/f sex. While socially conservative men might want to censor written f/f stories, they aren't going to cut off their own enjoyment of illustrated f/f sex.)
It's difficult to denounce something without first admitting that it exists.
Much of the criticism of women's reading of romance novels manifests in an accusation that too much reading of romance "ruins" women for real life relationships. The readers are encouraged to put the books aside and concentrate on their husbands and their marriages. The readers are told they will develop unrealistic expectations that can never be fulfilled and will be resultingly miserable the rest of their lives.
There is never any suggestion that what the heroines of Romancelandia are given by their heroes -- full measure of pleasure in their sexuality even when being raped -- ought to be and could be and should be the expectation of any woman who is the heroine of her own life.