While injecting drugs is not the only means of contracting hep c it is the most common. It only requires once even many years ago to become hep c positive that is a very common scenario.
While hep c is not generally considered by the CDC to be a sexually transmitted illness there is a small risk of transmission. The people at greatest risk for sexual transmission are those with multiple sex partners or those who engage in rough sexual practices or in the presence of the HIV virus. For long-term monogamous couples the risk of transmission is very low and the CDC does not suggest such couples need to take precautions to prevent transmission like the use of condoms or other barrier protection.
There have been many people at these forums married for many years where one partner has hepatitis C and the other does not. You should probably be tested to make sure that you don’t have hepatitis C but the odds are very good that you don’t.
As far as how he contracted hepatitis C it is very likely from his IV drug use in the past. Other than that from any other cause there’s really no way to know.