Why a "christian" conscience?

I hadnt' realized it had been a month since updating this. My apologies.

Still, I came across this video - http://www.upworthy.com/a-pastor-asks-a-politician-why-he-supports-gay-marriage-it-seems-he-wasnt-prepared-for-his-reply?c=ufb1 with an australian politician explaining how, as a christian, he can support gay marriage.

Frankly, I love his response. The pastor that asked him the question seemed wholly unready for his reply. And he makes many good points, such as that he can change because society changes - or, as he puts it another way, if they should get everything from the bible, slavery is a "natural condition" and the Confederacy should have been supported in the US Civil War.

I can't help but applaud that man.

However, one thing bugs me. He keeps talking about how he supports it with his "christian conscience." Now, I understand it being a sort of.. socially acceptable way of speaking, it's a nation with a christian majority among the religious and so forth.

But what he's saying goes against Christianity and the bible as he himself points out.

Why can he not just say "in good conscience" or "in good, moral conscience?" Or just say "It doesn't make sense?" Why should Christianity be brought in and given credit for something it's flat out against?

Admittedly, and rhetorically, it does avoid the "Christian vs Non-Christian" argument that could be brought up, as he's saying he approached it from his Christian perspective, or as a Christian. But it's not a Christian point of view in the least, strictly speaking.

How long before we can just say it's the correct, moral thing to do without having to refer to religion - especially with the assumption that religion is actually the "good" side of the argument?