Respond exactly as you just said: It’s ridiculous. To compare the zygote to egg and sperm is silly, because they are two completely different things. One is a fully merged egg and sperm containing all 23 pairs of chromosomes that determine sex, hair color, eye color, height, etc, and is growing and developing into a new fully formed human being. The other is a sex cell which, without it’s other half, has absolutely no chance of creating life and growing into a fully formed human person.
Alternatively, you can turn it back on them and say that if abortion is not murder, then neither is infanticide, neither is the killing of someone in a coma, neither is the killing of the elderly, neither is the killing of whatever arbitrary definition of “person” or “human” they want to make.