I am not sure where you got the 1:1 ratio at age 20. In your experience do 5% more men than women die before age 20. The Centers for Disease and Control keeps pretty good nmbers on death rates by age and gender. If you would like it in tabular form so it can be used in a spreadsheet you can go to the disastercenter web site (google it) on the bottom there in the small letters there is a link for US cause if death data. go to that page and follow the link to Number of deaths and death rates…
The information is also available on the CDC web site.
assuming a 3% OTD number with a 3:1 ratio we would have 2% more girls than boys . My issue with using this is that intil we have some form of hard numbers we are just guessing.
Re unmarrigable – You may be right that OTD/substance abuse is significantly more common among however phsychiatric conditions are significanlty more common among women. obviously most people with phsychiatric conditions can and will marry but it is another factor that somwhat cancels out the OTD issue.
RE the math – with a population on this size it is expected that there will be peaks and dips in the numbers. However there is an average growth rate that can be extraoliated from the dataset (with a certain degree of confidence). No one is saying everyone should marry x number of years older or younger than themselves. This would cause siginificant problems due to the year to year variations in the population. All we are saying is that due to the fact that the average growth rate is about 3.5% a community wide average age gap of 3 years is going to cause problems.