There is a pattern of declining sexual frequency in marriage which can be discerned from the sites of the Kinsey Institute and the Center for Disease Control (1982, Table E). (The data from Kinsey is more current.)
To summarize, the data indicate that currently married women were virtually certain to have had intercourse in the 3 months before interview, regardless of age, but their frequency of intercourse tended to decline with advancing age.
CDC (bold added)
Doh! You mean that the older I get the less frequent will be the sex? That’s the pattern for average men.
Men and women see sex differently. Men report a more accurate frequency (we can remember the last time that we had sex), while women report a frequency based on their feelings. Women generally don’t remember the last time that they had sex. With women, the frequency of sex is always adequate, while not so for men as the marriage continues.
What can I do to make sex more frequent? Keep track of when you have sex on the physical calendar that you both use and is hanging in a room you both are in frequently, but don’t tell your wife that you are keeping track of sex. Let her figure it out for herself unless she asks what the marks are for. Keep the marks small–you don’t want to send a signal to your wife that is try-hard. I use a small grin, like a shallow ‘u’. Also, don’t mark the calendar immediately after sex. Wait a few days. This will have the effect of causing your wife uncertainty about how often you have had sex recently. She will worry that it has been too infrequent. All good stuff!
Also keep track on the calendar of the sex count for the last two months. You will be aware of a problem sooner this way and so will your wife.