One thing that popped out at me that I think needs to be clear is she does not support marriage equality because of religion. From the description:
"As a married person myself, I don't know what it's like to be told I can't marry somebody I love, and want to marry," she said. "I can't imagine how that must feel. I definitely think we should all have the right to love, and love publicly, the people that we want to love."
There is nothing about religion in those remarks.
I had to go straight to The Independent article to which they linked to actually find where the religious part supposedly comes in.
She said, however, that her liberal attitude towards same-sex marriage comes because of her Christian values, rather than in spite of them. Though raised a Baptist, a church that tends to oppose homosexuality, Underwood and her husband Mike Fisher, a professional ice-hockey player, now worship in a non-denominational congregation.
"Our church is gay friendly," she said. "Above all, God wanted us to love others. It's not about setting rules, or [saying] 'everyone has to be like me'. No. We're all different. That's what makes us special. We have to love each other and get on with each other. It's not up to me to judge anybody."
Emphasis mine. She says that, but her other quote says otherwise. There's that phrase, "Actions speak louder than words." I find that sometimes some words speak louder than other words, too. I find that people are more honest about what they believe when they speak outside the context of religion.
There are giveaways in the later quotes. For instance, this silly idea that we are not to judge. It's not well thought-out. I find it difficult, though not impossible, for a person to be in favor of marriage equality over such a poorly thought-out premise. The other part is that she and her husband go to a non-denominational church. This may indicate there were things about the Baptist church she didn't find appealing, so she found a church that was more appealing. If so, this means she's finding a church that fits in with her preexisting views as opposed to forming her views around her church. Since the views came before the church, the church cannot be the reason for the views. Things that are causes must come first. That's not to say her views on gay marriage worked out the same way, but it does make me more suspicious since Baptist churches do not have a reputation for being pro-LGBT.
The last giveaway is that her logic doesn't really work out from a purely religious standpoint. She says, "We have to love each other and get on with each other." How does she go from there to (paraphrasing) "therefore I support marriage equality"? How does she know that supporting marriage equality is the loving thing to do? Maybe opposing it is the thing to do? That's where the first quote block comes in. That is her actual reasoning for her support.
The religious component, then, is just telling her that she should be a loving person. When you think about it, that seems a bit scary. She seriously needs religion to tell her to "love each other"?!? So, if she weren't religious, she'd be a hateful person instead? Actually, I doubt it. So, really, religion doesn't seem to play a part in her position whatsoever. What I think is really going on here is that she's trying to find ways to make her religion compatible with her marriage equality views.
Putting this particular case aside, I see things like this quite often. I find it to be rather disappointing. Too often I hear people claim they hold a position because of their religion...and then they proceed to give non-religious (secular) reasons for their position. I remember a similar instance when the NAACP came out in favor of gay marriage. Their president (?), when asked a question on how he thought members of faith would take this, said that their decision was because of faith. And then he immediately started talking about justice and fairness. It was obvious to me that the decision had nothing to with faith.