Homosexual couples "want the respectability that is attached to marriage without intending to pay the price" same as the athiest couples. The price, in this case being, finding a partner of the opposit sex and having the discipline to learn to get along with them dispite your differences, as well as, of course, beliving in the God you are making these vows to.
That's an attitude -- that the love and commitment and discipline shown by gays (and atheists, and Hindus, and so on) is somehow of a lesser quality or importance than the same things shown by straight Christians -- that I'm afraid I find distasteful.

And while I don't know what CS Lewis's thoughts on gay marriage were, I do wonder whether it would alarm him to see that sentence of his which you quote above -- written, as your fuller quote makes clear, against infidelity in loving relationships -- used by you to demean perfectly committed loving relationships.

Incidentally, I completely agree with you that atheists who want church marriages do a disservice to both religion and atheism, and I even agree with you that if they're clearly not taking the theistic parts of the ceremony seriously, it makes you question whether they're taking the human-scale parts of it seriously either.

Peter