Embryo freezing often stirs up debate amongst those who are for and against it. The arguments against it come from those who object for individual, cultural or religious reasons. They may not agree with your own views, but if you're considering this fertility treatment, you should be aware of the ethical implications of the procedure regardless.
An ethical implication that most fertility treatments face is that they involve a scientific process and turn the natural and/or spiritual process into something in which doctors interfere. Some fertility treatments are more hotly debated in this manner than others, and embryo freezing, because it involves lab-fertilized and discarded eggs, is among the more widely objectionable.
During embryo freezing, multiple embryos are frozen. This is because multiple embryos will be implanted into you during your in vitro fertilization or zygote intrafallopian transfer or similar impregnation treatment, to increase your chances of successfully becoming pregnant. Due to this fact, fertility treatments have a notoriously high rate of multiple births, sometimes producing even more than three children at once. There may come such a time where, due to your own health risks, you choose to have a partial abortion. The abortion is, of course, a hotly debated ethical issue.
Even if you don't have to choose or simply do not choose to have a partial abortion, the fact is that embryo freezing will likely result in some discarded embryos. Your fertility specialist will freeze multiple embryos not only to implant more than one embryo during your impregnation treatment, but also in case you decide to come back one, two, three or more times to have more children in the future. If you don't choose to become pregnant again, the rest of the embryos will be discarded. And even if you do, there's the matter of selection.
Selecting the "Good" Embryos, Discarding the Bad
Another ethical objection to embryo freezing is that your fertility specialist will study and grade all of your frozen embryos to determine which ones seem the most likely to succeed. Ones that don't seem as likely to succeed are most often discarded. Some people object to this pre-natal human-led "selection" process.
Freezing for Convenience
Another frequent objection is when healthy women freeze embryos for their convenience, in other words, so that they can delay pregnancy until a more convenient time. Women choose to do this because at a younger age, eggs are more plentiful and healthier than at an older age. (However, there is no conclusive evidence that embryos frozen at a younger age and implanted into an older woman will produce children any less likely to suffer from health problems, although neither will it increase their risk.)
This is, however, sometimes less frowned upon when a woman or a woman's husband/partner is about to undergo treatment for an illness, such as chemotherapy, and the woman or couple wants to freeze some pre-medical treatment embryos that may be more likely to produce healthy children.
Understanding the ethical implications of embryo freezing even when you decide to go ahead with the procedure can help you make the argument that you have considered both sides of the issue and you think your decision is an informed one. If you prefer to not have to justify your reasons, just be select about the people with whom you share news of your fertility treatment.