Hmmm, just following the thread so far.
I don't think you are unreasonable to want sex more often.
I also don't think it is unreasonable for a 51 year old woman with a 7 year old child to be content with once every three weeks.
No one is wrong. There is simply a miss match of sex drives here. So the options are: obligatory sex (basically rape) on her part or frustration on your part.
Neither of these seem satisfactory. So the question is, if you leave your wife, will your lot improve? You'll break up your family and may or may not find another partner who wants a higher frequency of sex, in the long term (everyone is keen year one, how does it look year five?)
I am a patient person used to compromise. If it were me, I wouldn't destroy the relationship by badgering her. If bide my time, I'd look forward to every three weeks. And I'd work to stay emotionally connected. A lot of couples have a renaissance after the DC get older and they have fewer responsibilities and more time for each other.
(Talking to a lot of friends/other women, I actually think you aren't doing too badly. Sex once or twice a month is a lot more than some couples, where the wife is going through menopause and still has young children. Comments on anonymous forum aren't necessarily broadly representative. Same as what I have picked up through tittle tattle!
)