You’re quite right: it indeed wouldn't be any different, IF we had arranged for this child to be conceived for the express purpose of being placed for adoption.
To begin with, we need to analyze the moral question in terms of ends and means.
The end of getting a child may be entirely innocent, even laudable.
But that end does not justify any imaginable means. Just to cite some obvious ones, you would agree, I presume, it wouldn’t justify rape, or prostitution, or concubinage, or infant-abduction, or cloning.
So even if the “end” of getting a child is praiseworthy, the means have to be looked at independently, to see if they are in keeping with the full truth of sex and marriage, and the dignity of the child as a person.
For instance, if we had decided to pay somebody to get pregnant so we could subsequently adopt the child, this would be getting a child (a good end) through an intentionally planned commercial transaction in which his mother sells him for money (a bad means).
Worldwide, this is regarded as an abuse and a crime. International adoptions were actually shut down for a period of time in Guatemala, for example, because women were being paid to get pregnant and then deliver their newborns to shady adoption agencies catering to European and American couples who were willing to put up big bucks and not ask questions.
In contrast, if a couple adopt a child who --- not by their arranging it so, but by whatever happenstance --- lacks the care and providence of his own father and mother, they are responding to his need without in any way having caused it.
Similarly, it’s wrong to beget batches of offspring and then freeze them for future implantation, adoption, sale, or experimental jiggery-pokery. But if this frozen embryo already exists, and the two existing options are either (1) adopt him and have him implanted in your womb, or (2) dump him down the sink, then adopting him is a praiseworthy thing because you are responding to his desolate situation without having in any way caused it.
Getting back to the related question you brought up: it’s necessary to make a distinction between fertility treatment, properly defined, and artificial reproductive technology.
Fertility treatment--- meaning treating the underlying cause of your infertility and curing it so you can have children via sexual intercourse--- is a good thing.
In contrast, artificial reproduction does not address the cause of infertility, nor does it cure it. It doesn’t make it possible for you to have children via sexual intercourse. Thus it doesn’t heal fertile sex: it takes the place of fertile sex. Thus it can't be unequivocally called a fertility therapy.
I do not agree with your views regarding marital sex, contraception, "fertility treatments" or the nature and/or victim status of children conceived through in vitro. We can certainly agree that there are moral and ethical considerations with that tool that are not always respected. However, more people and physicians are becoming aware of these issues and are using them more conservatively.
As sad as these case may be, high multiple births, custody battles over frozen embryos and even the embryonic stem cell debate have given people an opportunity to think about the downside of using this tool carelessly.