Look at it this way, these people are essentially asking us to imagine that a zygote, embryo and/or fetus are capable of cognitive thought (which they are most certainly not); they are asking that we treat the "unborn" as a person. What is up with the whole fetal personhood bandwagon?
One cannot argue against abortion based on their own personal beliefs about "personhood" - there is no consensus within the scientific field and certainly not among ordinary citizens. Some base their opinions on religion and/or spirituality (I suppose I would be in that group) but nobody knows for certain, therefor, using this as a basis for an argument is a fallacy. Back to my favorite quote:
EVERYONE IS ENTITLED TO THEIR OWN OPINIONS,
JUST NOT THEIR OWN FACTS.