I've been reading Matthew Vines' God and the Gay Christian, which is a worthwhile read regardless of your position on the issue. It's basically a popularization of James Brownson and other scholars who are affirming of same-sex sexual activity (SSSA from hereon out).
Vines spends one whole chapter on celibacy. His main point is that celibacy must be a "voluntary choice" and "not an imposed requirement" (47). No one can tell someone else that they are called to be celibate. The reason behind this move is obvious: if SSSA is wrong, then celibacy is the only option for gay Christians.
There may be a way to make this argument, but Vines doesn't succeed for a couple reasons.
Second, Vines' position reflects the assumption that only something that we "freely choose" can be authentic, real, and possible. But what about the celibate Christian who lives their whole life open to marriage, perhaps even desirous of marriage, but who wholeheartedly embraces their part in the family of God as a celibate person? I imagine there are a good many single, celibate Christians who would resent being told that "real" celibacy is only for those who freely choose it, over against those who might choose marriage if they were presented with the right person at the right time. Are there two classes of celibacy? Those who freely choose it at some point along the line and those who embrace it as God's calling even if they may prefer marriage, given the choice?
One might even think about this "freely chosen" criteria in relationship to marriage as well. A good many people--perhaps even the majority of pre-modern Christians--do not simply "freely choose" to get married to whom they want, when they want, for reasons they want. From arranged marriages to marriages based more on economic survival than romantic inclinations, one could easily argue that personal choice or preference plays a very minimal role in these marriages. This may still be the case for Christian marriages in many cultures beyond the individualistic and industrialized West. And yet, those marriages can be good, true marriages that are icons of Christ and the church. The question is not whether maximal personal freedom was exercised in 'choosing' this path, but the extent to which the persons embrace their part in in a way that points to Christ.
Many Christians unfortunately see celibate Christians as somehow second-class because they are not married. That's wrong and unbiblical. So it's strange, but perhaps not unexpected, to see the pro-gay marriage movement reinforcing the popular but unbiblical marriage-over-singleness thinking.
It's worth remembering that we exalt as Lord a single, celibate Jewish rabbi who prayed, "Not my will, but Yours be done," reminding us that the question is less about what we freely choose and more about who we will freely obey.