I agree that in our present society, infanticide is frequently illegal. There have been societies in the past, however, where infanticide e.g. infant exposure was still ultimately a practice; wikipedia tells me that it was extremely common in the stone age. Presumably, it was tan accepted form of population control before easy birth control. Conversely, apostasy was illegal and disgusting in many medieval societies but more widely tolerated today.

So whereas your disgust (and our illegality) at infanticide today serves as a good starting point to thinking about its morality, I don't think it's infallible and certainly not a veto over Singer's reasoned argument.