In my case it was a bit of a boiling frog situation and overdosing on hopium.
Dating and first year of marriage (5-6 years) sex was 3-9 times a week when we were living together.
A year after marriage she wasn't as receptive and affectionate but we were still having sex a couple times a week.
After the first was born she had internal scarring so sex wasn't always comfortable and she was tired a lot so we were down to once a week.
After the second was born, I had just come back from a deployment and some of the scarring had been cleaned up by the doctor during the birth so we had a burst up to a couple times a week again. This lasted maybe 6 months or so.
Once the youngest was mobile she was almost always "too exhausted". I tried to remedy this by doing more. Basically from the time I got home from work until after they went to bed I was hard at it. It was "enough" sometimes but honestly not that often. We were having sex a couple times a month for a while. This gradually went down to Ovulation Day Sex
(TM). I knew with great accuracy which day of the month she was likely to be in the mood and there was a chance of sex.
This was when the hopium started kicking in. Once the kids are more self sufficient, it will be better. Once the kids are in school, things will be better. Once I can work from home, things will be better. Once she's out of school, things will be better. Once she finds the job she wants, things will be better. For the last 5 years of the marriage a steady diet of hopium kept me in a sex 10 times a year relationship. Now I have a girlfriend and its finally better

.