I don't understand that position for a couple of reasons. For one thing, why put these particular Christian beliefs above other tenets? Those who oppose President Obama and the Democratic party based on Christian principles seem obsessed with homosexuality and abortion, while seeming to ignore other Christian tenets - caring for the poor, for example. By my study, Jesus spent far more time attending to the needs of the disenfranchised and outcast than He did preaching hell-fire on those who lived contrary to what He advocated. Yet, I haven't heard many anti-Obama Christians expressing concern for the government's lack of care for the poor or disadvantaged. Instead, they justify support for a party that for most of recent history has had an every-man-for-himself attitude when taking social and economic policy positions.
For many reasons - particularly the variant beliefs among denominations - Christians should be wary of inviting politicians and government officials to make their own beliefs part of their policy-making. Could a Jehovah's Witness, Seventh-Day Adventist, or Mormon be sure that their Catholic representative's belief-supported policies would be in tandem with their own? Our beliefs should dictate our own behaviors. If you believe homosexuality is wrong, have your intimate relationships with someone of the opposite sex. If you believe abortions are wrong, don't have one. Enforcing religious beliefs should not be government policy and we shouldn't use our votes to encourage it.
The earliest Americans who fled Europe's religious persecution certainly understood the dangers of giving government leeway to base policy on any individual's or church's beliefs. We would be smart to not undo their efforts. While the size and role of government continues to be part of the country's discourse, Christians, more than anyone else, should want to keep government and their beliefs separate.