Single embryo transfer, like all fertility treatments, carries with it a number of ethical implications. Even if you disagree with all of the ethical implications, you should be aware of their existence so that you can prepare yourself for encountering people who share these views of your pregnancy.
Doctor Intervention in Pregnancy
Some people object to any doctor intervention in the pregnancy process because they don't like the idea of pregnancy being a "laboratory experiment." Others object to certain levels of doctor intervention, but not to any and all fertility assistance, as some procedures are far less reliant on a doctor's guidance.
Single embryo transfer requires about as much doctor intervention as possible within the pregnancy process because your fertility specialist will oversee fertilization of your eggs within a laboratory. (Not all fertility treatments involve laboratory fertilization.) And, even more so than some other fertility treatments, the doctor will then select which single embryo seems the most likely to succeed, effectively leaving the choice of which child you will bear in the hands of your doctor.
Discarding the Rejected Embryos
More so perhaps than any other fertility treatment, single embryo transfer potentially involves discarding embryos. In traditional in vitro fertilization, your fertility specialist will implant multiple embryos in order to increase the chances of at least one embryo successfully implanting into the uterus. (This does, however, significantly increase your chances of multiple births, which the single embryo option is intended to resolve.)
In the single embryo variation, you will have no use for the many other embryos your fertility specialist has created in the laboratory. These leftover embryos can be frozen for future pregnancies--although all will still not be implanted--or they may be discarded, particularly if they don't seem likely to be successful implants in the uterus.
Increased Potential for Multiple Births and Partial Abortion
A frequent ethical implication of fertility treatments is the increased potential for multiple births. This is largely because in many variations of treatment, your doctor will implant multiple embryos at once because not all embryos (the doctor assumes, based on past data) will successfully implant into the uterus. However, they can. Having twins is typically safe, but having triplets or even more children can put both your and your unborn children's health at risk. You may then be faced with the challenge of choosing potentially developmentally disabled children, along with great risk to your health or partial abortion.
However, the entire point of single embryo transfer is to significantly decrease the risk for multiple births and attempt to bypass this ethical objection. While you will still have a greater risk for multiple births than a traditional pregnancy (because even the single embryo will have an increased risk of splitting into multiple identical children), you will know going in that with only one embryo implanted, you will have no fraternal multiples and a much smaller risk of multiples at all.
Only you can decide if single embryo transfer is the right choice for your pregnancy. This fertility treatment has been responsible for many successful pregnancies in women and couples who could not otherwise bear children, and there are some less ethically dubious aspects of the procedure over some alternative choices.