At his blog, Thomist philosopher Edward Feser recently wrote the following: “Sexual desire is extremely powerful and the demands of sexual morality an especially irksome imposition on the will. Hence the tendency of liberalism is to try as far as possible to eliminate or at least soften and minimize the importance of such demands.”
But who cares what liberals are trying to do in terms of eliminating or softening sexual moral demands?
The questions that should not be lost here are these:
(1) Are the sexual prohibitions placed on people justified (from masturbation, to contraception, to gay marriage)?
(2) Are Thomists begging the question when they essentialize marriage as centered in reproduction, raising children, and promoting family?
(3) Can marriage be redefined under Thomist assumptions in a way that is oriented toward love as the essential core of it? And if it can be, why shouldn’t it be done?
The focus on liberalism and the Church’s inside baseball in Feser’s post distracts from a direct grappling with these questions. If you oppose sex for pleasure and gay marriage, you need to justify those positions, not obscure the issues by gaming the motives and politics surrounding the debate. That’s blue pipe smoke cast over the intellectual chessboard.
Prometheus Unbound 
I believe you are correct in asking for justification and spinning that backward at the Thomists. in the end all social demands become political. Many theists seem to live in hope that human societies will simply act out of tradition rather than reason so that if they can establish a tradition of sexual morality none will question it. Many of the arguments in support of moral demands are in fact argument from tradition.
Most human traditions have lost the source of origin and original purpose but are touted as necessary for society. To see what I mean, when you next sneeze in public monitor the reactions closely. In westernized or Christian societies you will almost always hear “bless you” – where does this come from and is it rude to not say bless you when someone sneezes? Ask people why they say ‘bless you’ and you are likely to hear some very odd answers – most of them falling to ‘because it’s polite’. So we have many traditions which have no valid or recognizable origin and this is what theists hope will become of their moral demands.