[K. in Cleveland]

OK, I want to know if this is a new thing or if it has always been like this... Also does it hurt when you have intercourse? If it does hurt you during intercourse you may want to see a urologist as there may be a medical condition.
If sex is pleasurable for you and it is only when your wife performs oral sex then it's more than likely her technique. Honestly most women don't really know how to perform fellatio when they're new at it. It could be that she's not careful with her teeth, or that she's being too rough and maybe bending or folding the shaft which could cause discomfort.
There's also a possibility that you have an infection or STD... in which case oral sex would probably not be the only uncomfortable act for you. If you can urinate, masturbate, and penetrate (the 3 "ate's" of sex) without any pain or discomfort the issue probably lies within your wife's performance, however, if any of those other things are uncomfortable, talk to your doctor. And don't be embarassed, medical people see people's most intimate parts day in and day out. It's not something they laugh about, make fun of people for, or judge someone about... it's simply another day at the office.