Because he can't and so he won't. The Catholic Church, on the other hand, has taken a strong stance against homosexual unions.
Homosexual "Marriage"
True marriage is the union of one man and one woman. Legal recognition of any other union as "marriage" undermines true marriage, and legal recognition of homosexual unions actually does homosexual persons a disfavor by encouraging them to persist in what is an objectively immoral arrangement.
"When legislation in favor of the recognition of homosexual unions is proposed for the first time in a legislative assembly, the Catholic lawmaker has a moral duty to express his opposition clearly and publicly and to vote against it. To vote in favor of a law so harmful to the common good is gravely immoral" (UHP 10).
UHP Congregation for the Doctrine of the Faith, Considerations regarding Proposals to Give Legal Recognition to Unions between Homosexual Persons
Catholic Ping
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They've always had a giddy and odd sense of humor on domestic and sexual issues. Isn't it up to the "Supreme Head" - Elizabeth Saxe-Coburg-Gotha "Windsor" - to render a decision on such ecclesial matters as who can get married and the qualifications for holy office? I mean...uh...let's keep with the program gang. Or shut up.