I think we need to differentiate your examples.
Human beings will very likely never have four arms, because our skeleton is not made to support four arms. It (very likely) simply won't work, from an anatomical perspective.
Such questions are non-sense questions, and I don't see their merit.
Asexual reproduction and test tube babies are a reality, on the other hand. While we don't know if such procedures will become widespread in the future, psychologists might play a crucial role in determining wether or not our society should support or ban them.
Psychologists are not prophets, but meteorologists or economists aren't prophets, either, and still they earn their daily bread by attempting to predict the future. Since most political decisions affect human emotional well-being, psychology as a discipline has the duty to attempt to predict these effects. We cannot let laymen decide the fate of a nation from an economic perspective alone.
Therefore, yes, questions about plausible future developments are definitely within the scope of our sciences and therefore ontopic. It does not matter that our disciplines cannot answer many of such questions at the moment, but I don't see why we shouldn't attempt to research and document the foundations that might have been already laid, and point out directions for future research in the same way that every journal article does.
My prerequisite for an answer to such a question would be that it is not merely opinion and speculation, but provides quotable theories or related research.
For example, the Wikipedia article on In vitro fertilisation ("test tube babies") summarizes the results of a study:
In a 2005 Swedish study, 166 women were monitored starting one
month before their IVF cycles, and the results showed no significant
correlation between psychological stress and IVF outcome. The study
concluded with the recommendation to clinics that it might be possible
to reduce the stress experienced by IVF patients during the treatment
procedure by informing them of those findings. While psychological
stress experienced during a cycle might not influence an IVF outcome,
it is possible that the experience of IVF can result in stress that
leads to depression. The financial consequences alone of IVF can
influence anxiety and become overwhelming. However, for many couples,
the alternative is infertility, and the experience of infertility
itself can also cause extreme stress and depression.
This is a perfect example of actual research into a future problem. It does not directly answer the question about mother-child bonding, but it does relate to that question (it as been intensely studied how postpartum depression affects the relationship between mother and child) and it is well possible that other psychological aspects of in vitro fertilization haven been studied but not documented on Wikipedia.