Over at First Things, this article describes one Christian's perspective on homosexuality and marriage. The author quotes Elizabeth Anscombe’s essay "Contraception and Chastity":
"If contraceptive intercourse is permissible, then what objection could there be after all to mutual masturbation, or copulation in vase in debito, sodomy, buggery (I should perhaps remark that I am using a legal term here—not indulging in bad language), when normal copulation is impossible or inadvisable (or in any case, according to taste)? It can’t be the mere pattern of bodily behaviour in which the stimulation is procured that makes all the difference! But if such things are all right, it becomes perfectly impossible to see anything wrong with homosexual intercourse, for example. I am not saying: if you think contraception all right you will do these other things; not at all. The habit of respectability persists and old prejudices die hard. But I am saying: you will have no solid reason against these things. You will have no answer to someone who proclaims as many do that they are good too. You cannot point to the known fact that Christianity drew people out of the pagan world, always saying no to these things. Because, if you are defending contraception, you will have rejected Christian tradition." (emphasis mine)
While the details will differ on reaching the conclusion Scripturally, there are good points made here. The answer to the questions of contraception (as well as homosexuality) rest solely in the question: "what has been given by God in Christ?"
Marriage is a given estate of God as gift. Luther highlights this in the Small Catechism in drawing out our gifts of "daily bread" to include devout husband or wife and devout children. The reality is that not all gifts are given equal save one. The gift of salvation, freely offered and given as complete and finished in the death and resurrection of Christ. The gifts of daily bread that follow vary among us.
The Christian reality is in the gifts given by God in Christ. Marriage seen as right and not as gift will have a differing view. Children seen as right or commodity or anything other than gift will have a differing view. Christians are indeed drawn out of the pagan world. Drawn out to receive the gifts of God in Christ. Gifts received in faith as the Lord gives. Important for us to remember that gifts given are simply gifts. They are given as the gift giver would have them be received. He gives marriage as one man and woman and within that union He gives children. We are called out of this pagan world to receive Christ's gifts. The gifts He gives from the cross as well as the temporal gifts. Not with greed, not with complaint, and not with demands of telling our Lord when and how to give. We receive as beggars, with joy, by faith that He indeed is Lord of all creation, over life and death, over marriage, over the gifts of creation. Thanks be to God.