It's against his religion.
Aside from McClurkin (whose views then-Senator Obama distanced himself from, but whose support he has continued to court and enjoy in a back-channel wa) and anti-gay zealot Rick Warren (to whom the President gave a national platform at his inauguration), there is also the Circle of Five. NYT:
President Obama has been without a pastor or a home church ever since he cut his ties to the Rev. Jeremiah A. Wright Jr. in the heat of the presidential campaign. But he has quietly cultivated a handful of evangelical pastors for private prayer sessions on the telephone and for discussions on the role of religion in politics.
All are men, two of them white and three black — including the Rev. Otis Moss Jr., a graying lion of the civil rights movement. Two, the entrepreneurial dynamos Bishop T. D. Jakes and the Rev. Kirbyjon H. Caldwell, also served as occasional spiritual advisers to President George W. Bush. Another, the Rev. Jim Wallis, leans left on some issues, like military intervention and poverty programs, but opposes abortion.
None of these pastors are affiliated with the religious right, though several are quite conservative theologically. One of them, the Rev. Joel C. Hunter, the pastor of a conservative megachurch in Florida, was branded a turncoat by some leaders of the Christian right when he began to speak out on the need to stop global warming.
But as a group they can hardly be characterized as part of the religious left either. Most, like Mr. Wallis, do not take traditionally liberal positions on abortion or homosexuality. What most say they share with the president is the conviction that faith is the foundation in the fight against economic inequality and social injustice.
That is, I suppose, one way of thinking about social injustice, although here's another one:
Rev. Jakes refers to homosexuality as “brokenness” and has claimed that he wouldn’t hire a sexually active gay person. But it seems T.D. can’t even keep his own son off the D.L. (down low). His “sexually broken” heir was arrested earlier this year for cruising a Dallas Park in search of gay men.
Wallis, the chief executive of Sojourners, a Christian magazine, holds “traditional” views on homosexuality and abortion, according to the Times article. Although Wallis has taken some affirmative steps on GLBT equality, he prides himself on not being a part of “the religious left.”
Rev. Caldwell has endorsed Metanoia, an ex-gay ministry designed to “help homosexuals understand with God’s help that ‘change [is] possible.” When the GLBT community worked to elect Obama, this is not what we thought he meant when he promised “change.”
Back to the NYT to find the President on the phone, praying with the man who considers homosexuals to be broken and admits he wouldn't hire one:
Bishop Jakes said he had been tapped for several prayer phone calls — the most recent being when Mr. Obama’s grandmother died in November, two days before the election. “You take turns praying,” said Bishop Jakes, who like the other ministers did not want to divulge details of the calls. “It’s really more about contacting God than each other.”
Truth of the matter: President Obama depends heavily on 95%+ of the African-American vote, as do most Democrats with national aspirations. And that African-American voting block, as a demographic, is significantly more homophobic than other groups.