I honestly can’t say I have a problem with selecting the healthiest embryo for implantation. Fewer than half of naturally conceived embryos ever survive until implantation, and a third of all implanted embryos end up dying. So choosing the healthiest one is an attempt to shift the odds a little.
I do, however, have a problem with IVF. When people can’t conceive naturally, there is a reason for it. Is it really wise to try to circumvent whatever problems are causing the infertility in the first place? Doesn’t that thereby preserve those problems which natural selection would normally weed out? Also, what rationale is there for trying to create babies who may have serious genetic defects (such as an inability to conceive naturally), when a million perfectly healthy babies are being killed every year?
The problem is that we are not God and need to be careful to not do what only He, as God, has the right to do.