A lot of the discussion above seems confused by the fact it’s been carried on with the blunt category of being “morally opposed” to public policies. Let me try to sharpen up this concept before giving an opinion.
First: does “morally opposed” mean you think something is wrong? The answer is no, because, for example, it is wrong not to brush your teeth regularly, but it should not be illegal not to do so.
Second: does “morally opposed” mean you think something is intrinsically evil? Again, the answer is no, because otherwise our society would have to outlaw fibbing.
These first two questions show that we need to get outside moral theological concepts in order to understand how our morality should inform our voting. In particular, they show that it is fallacious to argue that “X is wrong, so X should be illegal.”
From these preliminary remarks, we can go on to attempt a general answer to the OP’s question. Public policy is normally intended to foster the common good either directly or indirectly (i.e., by advancing the cause of justice or inhibiting injustice, or by adapting means to already given ends). Moral beliefs should give us a sense of what the common good consists in, and they should help us to sort out what is just from what is unjust. Thus, abortion is not only wrong (a fact of almost no political significance) but it is a grave injustice and therefore ought to be illegal. Or again, as the Church teaches and we are bound to agree, gay “marriage” is incompatible with the common good in one way or another. Therefore, a Catholic cannot vote for it. (NB: Stated flat out just like that, this teaching strikes me as cruel. In societies where gay “marriage” is a live option it should be pointed out that the Church teaches that some kind of couples registry to achieve the same material benefits would be perfectly acceptable.)
So my answer to the OP is that the issues he raises are essentially philosophical, and he could do a lot worse than trying to work out the answer for himself by reflecting on the ends that morality and politics serve. From this point of view, his best bet would be to start reading Plato’s Republic.