Hello. Female nurse here. I understand your concerns, but at work, yes you are a nurse first and a male/female second. That is part of being a professional. Believe me, I don't mind to help a fellow nurse out but sometimes I simply don't have time to stop what I'm doing with my own patients to go catheterize someone else's patient. Catheterizing females is not always easy and can be time consuming.
All nurses should be capable of doing their own procedures because someone may not always be available to do it for you. And saying you choose not to do a procedure that is part of your job does not sound very professional.
As long as you explain the procedure to the female patient beforehand and explain everything that you're doing during the cath, having a female nursing assistant in the room for a witness, it shouldn't be an issue. Sometimes females refuse to let males do this procedure. In that case, yes, you shouldn't proceed and should tell your charge nurse and have a female do it. But it shouldn't be a case of the male nurse refusing to do it because you are uncomfortable with it. And not all females are as modest as you may think. We are used to those yearly gyno visits.