We often see references to sexual problems in women, but men have them, too. There are many medical issues that can cause sex problems, as well as emotional or psychological issues.
Generally, problems fit into four categories:
Desire or libido
Problems with orgasm
Pain during sex
Desire or libido issues can be related to fatigue, suffering from depression, or low levels of hormones such as testosterone. If the problem is hormonal, it can often be discovered with a blood test, and hormones can be replaced. Reading about women who are with men who don't seem to have a sex drive reminded me that there is sometimes a simple fix.
Men often don't like to admit to a libido problem, because in our culture they may feel like less of a person when they discuss these issues. But if the problem is something like low testosterone, a health care provider can do a blood test, prescribe testosterone which often comes in a gel form he just rubs on his skin, and a difference can be seen within a couple of weeks. The change can be pretty dramatic.
Don't assume something is wrong with you if you have a difficult sex life. Sometimes the problem is a partner mismatch, but sometimes it is a medical issue that once resolved can return sex life to normal.