Something is wrong with my knee and I'm only 41! who do I see first? Having tough time in the sack.

Primary care phys. I'd recommend to see a primary care physician. Sometimes the problem can be resolved at this point. But sometimes he/she needs to refer you to different specialists and that depends on the type of symptoms/signs he/she can find during the encounter. This specialist could be an orthopedic surgeon, a rheumatologist, a podiatrist, etc. Depending on the case.