An Alternate Christian Opinion on Gay Marriage

Several months ago I had a brief but interesting conversation with my younger sister on the topic of gay marriage. Along with the rest of my family, she is a practicing Catholic who accepts the teachings of the Church, to the best of my knowledge, on all topics. She mentioned, however, that there was a petition after Mass about making gay marriage explicitly illegal in Florida which she declined to sign and my mother, who was with her, asked her why. She said that it was none of her business who anyone else wanted to marry because it didn't affect her at all. That particular sentiment is fairly common among moderate and liberal Christians, but what she said next was more novel. She said that pre-martial sex is considered just as sinful as homosexual sex, but no one was trying to make fornication illegal. She understands that you can't legally prohibit something if it doesn't hurt anyone just because you think it is sinful. If only more Christians in the United States shared her simple appreciation for separation of church and state.