Suppose that you and your wife become alarmed by the fact that Junior is starting to do some bad things that you never had to have explicit rules about before. In particular, he's doing X and Y, which you quite reasonably regard as species of the same wrong act. In fact, if you prohibited someone from doing X, he'd probably assume he was also prohibited from doing Y unless you expressly stated otherwise. Your wife thinks that Junior should be stopped from doing X but should be allowed to do Y. You and she go back and forth for a while and can't come to any nearer agreement. Meanwhile, Junior is merrily going on doing both X and Y without the slightest fear of reprisal, fear of God not being Junior's strong suit, and fear of man (namely, his parents) not having been brought to bear on the situation. It seems to you rather important that he should be stopped, right away, from doing as much of this stuff as possible. So after one more conversation with your wife, you go to Junior and give him the following speech:
Son, we've not been stopping you from doing X and Y, but that situation is about to change. From now on, if you do X, you will receive such-and-such a punishment. You are hereby forbidden to do X. Unfortunately, your mother and I cannot agree about Y, so for the time being you are not threatened with any punishment from us for doing Y. This doesn't mean that Y is not wrong or that you shouldn't be punished for doing Y just as much as for doing X, and the situation with regard to Y may well change later on. Meanwhile, we're going to make good and sure you get in big trouble if you do X. Got that?
Now, I don't think anyone could say that you were authorizing or endorsing your son's doing Y. Nor does there seem to be any important distinction here to be made between the "author" of the prohibition thus stated and its other supporters. In this case, even though you are the one who wants to prohibit both acts, you are the "author" of the speech to Junior, and your wife is its supporter, rather than vice versa, but it seems to me obvious that that fact does not put you in the wrong. That is to say, you would not be in any morally better situation if your wife made a parallel speech to Junior and you "voted" for it or endorsed it in some other way. In fact, by putting yourself forward as the "author"--the one actually to speak to Junior--you can be especially careful that nothing that even looks like an endorsement of the moral licitness of Y gets into the speech, and you can warn Junior that his days of freedom to do Y may be numbered.
I think this is actually quite a good analogy for the situation of a pro-life legislator who gets an opportunity to write and propose a law outlawing abortion in some cases but allowing exceptions in others, where such a law will vastly improve the legal protection for the unborn in his jurisdiction, and who does so.
It might also be worth pointing out that the heinous and iniquitous court decision Roe v. Wade is not analogous to either of these things--to the pro-life legislator writing a law with exceptions or to the parent speaking to his child. For Roe really did attempt to lock the states into a situation where they could not protect unborn children, and it did so by saying (which was patently false) that abortion is a constitutional right. In so doing Roe's intention and effect was to strike down state laws that protected the unborn. Roe in that sense really did "authorize" abortion in a very specific way, and it is to Roe most of all that we should apply statements about the evil of "laws" (or in this case, the quasi-laws that are court precedents) that authorize abortion.
See here and here for the background to this post.