Why do voters in a presidential election vote on moral issues? When casting a vote in a presidential election the deciding factors should be platform issues that the executive branch of the government has actual direct influence over. The president has direct influence over the economy. The president has direct influence over foreign policy issues. The president has no direct influence over whether abortion is legal or whether a gay couple can get married. Those issues are constitutional issues that sit firmly in the judicial branch. The only real influence the president has over those issues comes very indirectly from the president's ability to nominate a judge to the Supreme Court. Now, to be sure, a conservative president will nominate more conservative judges, a liberal president more liberal ones. But there is no guarantee that a judge will retire during a president's term nor is there any guarantee that even the most conservative judge will vote conservatively on the court (the Constitution is a very liberal document, after all). Justice Warren was an appointee of Eisenhower, a Republican, and ended up being one of the most liberal justices in history. Souter also was a Republican appointee (George H.W. Bush) who became a liberal justice. So given that the president only has indirect influence over the judicial branch through appointments of judges and then has no control beyond that of the judges' decisions, it would seem logical that issues that are decided by the courts would be very low down on the list of issues that voters decide their presidential votes on.
Yet this is not the case. In fact, moral hot-button issues like abortion and gay marriage are often the main issues that voters decide on. Economy and foreign policy very often take a back seat to these moral issues. Why? Because people are not rational decision makers and they tend to vote emotion rather than logic. In fact, when high emotion gets triggered, as it will when confronted with these moral issues, the frontal lobe of the brain actually shuts down. The frontal lobe controls rational thought. The limbic system controls emotion. If the limbic system is lit up, the frontal lobe shuts down. The two types of thinking, emotional and rational, really can't coexist. In poker, when emotional thinking takes over, it is called tilt. Tilt is the state of being so emotionally overwrought, usually because you have had some very bad hand happen to you that you lost, that you can't think straight and start making really bad decisions (generally losing a lot of money in the process).
Sound familiar? It doesn't just happen in poker. Have you ever heard the aphorism, "Why don't you sleep on it?" or "Take ten deep breaths before making a decision"? These sayings are meant to get you to stop and wait till your emotions have calmed down before acting and making a decision because emotional decisions tend to be irrational decisions since rational thought can't really coexist with a lit-up limbic system.
So it turns out, that voters in presidential elections tilt just like poker players. Whenever a voter casts their vote for a candidate that will hurt their economic interests just because they are pro-life or anti-gay marriage they are on tilt -- just as if they lost some really ugly hand at the poker table. It is not that a voter is not allowed to want a candidate's morality to line up with theirs. It is just that the moral issues should not be the driving factor to the decision. The planks in the platform that sit firmly in the control of the executive branch should drive the vote. But it seems to getting rarer to find voters who use that strategy.