I slept with a girl july 1st - the 4th, unprotected. first she said her due date was march 21st and i could be the father. now the due date is march 1?

Depends on accuracy: The question is how accurate are the due dates. The are very accurate if determined by ultrasound when the woman is 8-11 weeks pregnant. If the due date is correct, march 21 matches well, but march 1 doesn't match well. A potential father can also wait until the baby is born to see who the baby looks like (the old fashioned way), or do dna testing (the new way).