How can baby May not have happened yet when stories about baby May have already been done in the original timeline?
OMD erased baby May from existence along with the marriage, before that baby May and the marriage existed.
In this new timeline baby May never existed and she will never exist in the future either because she was meant to be born in the past during the timespan of the second clone saga. Any future child that may end up being born should Peter and Mary Jane get re-married in the new timeline won't be the original baby May...the child can be given her name, but she won't be the original baby May that got erased from existence via OMD.
If someone went back in time and prevented one's conception then that sperm and egg that was meant to be that person never came into contact with each other to form that person and as a result will never exist unless the original timeline is restored.
That's what happened to baby May due to OMD.