like many who have posted, i have experienced girls of all "tightness" ranged in their age and parenting, but it seems like some put more emphasis on muscle control.
Definitely a myth, natural childbirth/vaginal childbirth is a wonderful experience and does not stretch you out or leave you loose. Whomever says that is just not educated on the subject. All my births have been vaginal and mine is very tight. Its all about the P muscles! :-)