Let me put a few things on the floor:
I'm one of those Christians. (Actually most of the time I don't even like it when people call me a Christian because I'm afraid they might put me in the same box as that guy who wrote that article. And Lord have mercy if people think I'm like that.) I'm one of those Christians that supports equality. Period. Marriage equality, gender equality, pay equality, ethnic equality. I just think equality is groovy. And at the heart of Christianity.
If you're using the Old Testament to defend yourself in your bigotry against homosexuality, I'll argue this: "the Bible is the cradle wherein Christ is held" (Martin Luther). Anything in the Bible that goes against the teaching of Christ (who is the Word), is simply invalid or misinterpreted. Yeah I actually believe that.
If you're using Paul to defend yourself in your bigotry against homosexuality, I'll share this: Paul was kind of a jerk. Also, he was writing to first century converted Christians- not middle-class American Christians in 2014. So don't take every single thing he said as the cold hard truth. Just chill out a bit. Start quoting Jesus a little more than you quote Paul and then maybe I'll take you seriously.
But I digress; let me get down to answering these questions.
1. On what basis do you still insist that marriage must be monogamous?
Biblically, I don't really have an issue with people having multiple spouses. In the United States, however, we have laws that won't let us have multiple wives/husbands (darn). As a Lutheran in the ELCA, my denomination does indeed state it must be a monogamous relationship in order to be recognized by the church. Culturally (and historically in the US), people enter into marriage with one person and not multiple persons.
So I guess my answer is, as a Christian speaking not for my denomination but for myself and my own convictions: I don't insist that marriage must be monogamous. Need citations about when polygamous relationships worked out alright in the Bible? Genesis 4, 1 Kings 11, Abram sleeping with Hagar...
And to the comment about sons and daughters and essentially incest? He cites medical information later, but ignores it here. Products of incest are more likely to be born with disabilities or medical problems. Plus, it's a law in the US (mostly for this reason).
2. Will you maintain the same biblical sexual ethic in the church now that you think the church should solemnize gay marriages?
Uh, yeah. When gay marriages fall apart, we will mourn as a congregation for that loss. I still think sex before marriage is something to be taken seriously. Do we insist on life-long committed monogamous relationships with heterosexual couples? Are we just as completely appalled when a pastor has an affair with another woman? Well yeah. Because there's commitment and fidelity there. And when you break that commitment and trust, you're hurting everyone. If a gay couple is struggling with fidelity and the family is being hurt by it, then I would handle it the exact same way as if it was a heterosexual couple.
3. Are you prepared to say moms and dads are interchangeable?
What? We are supportive of single moms, single dads, divorced parents. Actually we are probably more supportive as a church of these groups than "happily married with 3 kids mom and pop." Why in God's name wouldn't we be supportive of two people who love one another and want to raise a child together? My church and religious community is very much into it taking a village to raise a child. This argument is completely invalid. Sometimes my parents interchange their usual gender roles. Does that mean I'm deprived of something? Hardly. And let's face it, no matter how you raise your kid, there's a really good chance they'll need counseling in their mid-twenties anyway.
4. What will you say about anal intercourse?
Um, nothing. Are pastors actually preaching about anal intercourse? Because I'd love to hear that sermon spun into the good news of Jesus Christ. I don't preach about other things that cause health problems either (smoking, drinking, obesity, microwaves), so why would I preach about that? Plus the fact that heterosexual couples are just as able to have anal intercourse as homosexual couples and no one has said that in all this time. If someone were to ask my "Christian opinion" on it, I'd tell them that my body is a gift from God (as is my sexuality) and I plan to treat it as such and I hope other people treat their bodies the same way.
5. How have all Christians at all times and in all places interpreted the Bible so wrongly for so long?
To quote my good seminarian friend, "because we were dead on with that slavery thing too." Is this a serious part of the argument? Because it kind of seems like a joke. People also spent a zillion years interpreting the Bible as literal truth. We know now that that is not a great way to interpret the whole Bible. Well, okay, most of us know that.
"The church has been of one mind on this issue for nearly two millennia. Are you prepared to jeopardize the catholicity of the church and convince yourself that everyone misunderstood the Bible until the 1960s?"
Uh, yup. That's pretty much exactly what I'm prepared to do.