Since MP Pat O'Brien is a Liberal (former) that opposed gay marriage:

from The Liberal Case Against Gay Marriage in The Public Interest:

The issue of gay marriage brings to a head, like few other issues of our time, a central conflict between two moral positions that interact like seismic plates beneath the surface of contemporary American political life. It is commonly thought that the issue of gay marriage pits secular liberals against religious conservatives. While this understanding is accurate up to a point, it is also seriously misleading. The most stubborn and intransigent opponents in the conflict are both in their way sectarian.

The first position is more or less a traditional Christian one. That is, it accepts the idea of an authority higher than human choice that must remain within limits set by that authority. New understandings of these limits have arisen in recent years, allowing the individual pursuit of happiness more leeway and removing much of the shame and guilt that once kept traditional sexual norms in place. Nevertheless, its basic familial ideal remains intact: a monogamous, heterosexual, and devotional relationship directed toward the rearing of children. For most proponents of this view, gay marriage represents a direct assault on the grounding authority by which life at its most serious and intimate is lived.

The second position, which takes human freedom as its central and highest good, could be classified as “liberationist” or postmodern. Distrustful of traditional rules as intrinsically oppressive, it seeks the individual’s emancipation from all norms that might hamper the quest for spiritual and material autonomy. For the most radical liberationists, all universal norms are suspect, with the sole exception of something like a duty to “accept difference.” Among the more moderate proponents, this suspicion is replaced by an uneasiness with respect to “moral judgment” that approaches or imitates humility of a more traditional Christian sort, at least when applied to others. Thus, for the liberationist camp, gay marriage is either a celebration of the individual’s heroic struggle to find love and validation in a hostile world, or at the very least, it is no one else’s business.

The debate over gay marriage is currently polarized by these two sectarian forces. It would be politically beneficial to define a genuinely liberal approach that is fair to both.

[for commentary at Alas, a blog]