This.
As someone who would apparently be given priority in Portugal, I would appreciate this in situations when it is obviously needed. While it's mostly common sense and common courtesy (and no, I don't think either should have to be legislated, but I don't write the laws in Portugal), unfortunately it can't be assumed that everyone possesses either of these qualities (see post #6), and while I would love to appeal to ideas of 'fairness' and 'equality' when it comes to most aspects of how society should work, if it is simply a fact of life that we are not equal as human beings (and at the level which the law seems to address, it is), then why should anyone pretend otherwise? After all, from the description of what the law entails, it seems like it is mostly designed to help facilitate these peoples' participation in commerce, which is something that everyone involved should want to help realize. The money of the disabled, the pregnant, or the elderly is still money, after all, and these people no doubt want to participate in society to the fullest degree and as efficiently as they can. I can't see what sense there is in being against that in any way, other than perhaps the usual presumption of the able-bodied, young, and/or not-pregnant (not all of them, but certainly of those who would fail to see the usefulness of such laws) that people who are in such states ought to be treated the same as those who are not because that is what is 'fair'.
No it isn't. It just isn't, and nobody sincerely thinks that it is, and it's fairly easy to prove that: our national armies are not full of the elderly, the disabled, and the pregnant, and nobody on the pseudo-fairness flotilla would argue that they should be just because those who are not in any of those conditions are thereby made to fill up the ranks. Similarly, nobody ever seems to argue that the recommendations that the pregnant not ride roller coasters, drink alcohol, or smoke cigarettes are somehow 'unfair', even though these are things that those who are not pregnant can do to their heart's content (seems pretty 'unfair' to me, if we're going to treat everything as though it occurs in a vacuum). But suddenly when it comes to people who have no real problem that prevents them from being able to stand in a line at a shop potentially having to do so for a little while longer than they otherwise would so that the little old lady in line can buy her milk and get on with her day before others (who would anyway take half as long to do so as she does) can do the same, it's some horrible affront to fairness or equality or dignity or whatever.
Come on, really? Sure, I guess if the stores make some huge deal out of it by cordoning off those who need help from the rest, or ringing some big alarm whenever there's a pregnant woman in line or something, then I could see that being the case (I'm just one person, but I don't particularly want to draw attention to myself when I am just going about my day; I would certainly rather the world not stop on its axis every time I need to buy paper towels or whatever, so I don't know where this idea of 'entitlement' comes from, beyond recognizing that, yes, I ought to be entitled to participate in the outside world to the degree that I can and am actually putting myself out there for the express purpose of doing so; I'm sorry that this apparently comes across as arrogant to some people, though I honestly cannot understand why). But that's not what the law entails, from everything we can tell by reading the OP.
So I would say that if this law is enacted in a sensible way, it would probably be to everyone's benefit.