Why it's difficult for Christians to be persuasive on SSM

I think our ongoing conversation on MoJ about same-sex marriage is helpful and fascinating, perhaps because it tends to proceed with less shrillness and venom here than it does in most other venues today.  I just want to point out two reasons why the case against SSM might be such a difficult one for Christians to make persuasively, even putting the merits of the argument to the side:

First, Christians in general have been much more outspoken about SSM than about non-SSM threats to the sanctity of marriage.  Last summer I spoke to a group of conservative evangelical Christians about SSM, and this is the image I used to convey the GLBT community's distrust of Christians on this issue:

Imagine that marriage is a house, and the Christian is sitting on the front porch.  The house is engulfed in flames.  A gay person is walking down the sidewalk, lighting a cigarette with a match.  The Christian stands up and yells, "Hey, don't throw your match near my house -- that's a fire hazard!"  Viewing the scene, the gay person can't help but conclude: "This isn't about marriage.  This is about me."

Second, over the past fifty years, very few Christians have taken leadership roles in condemning obvious injustices against the GLBT community.  Instead of letting Anita Bryant, Jerry Falwell, et al. define the "Christian" perspective on the law's treatment of homosexuality, what if more mainstream Catholics and evangelicals had been outspoken regarding job discrimination, harassment, and violence targeting gays?  It became impossible to separate bans on interracial marriage from the scandalous history of race in this country.  It is becoming very difficult to separate bans on same-sex marriage from the scandalous history of homosexuality in this country.  (Note that I'm not equating the merits of the arguments in support of such bans; I'm simply saying that the historical contexts of the bans are leading the public to embrace similar conclusions regarding their rationales.)  The Church has tried to separate the recognition of homosexuals' dignity from the marriage issue, but that's not easy.  If the public rejects the notion that homosexuality is an "instrinsic moral evil," will the embrace of SSM be far behind?  Christian history has made it too easy for observers to conclude that opposition to SSM is part of a rearguard action by Christians who are perceived to have pushed to marginalize gays and lesbians at every turn.  Under these circumstances, it's very difficult to persuade the GLBT community that the debate about marriage is about marriage, not about them.