I know your problem - My boyfriend doesn't want to get his male dog fixed. I think it's a guy thing. "How would I feel if someone cut my balls off?" Kind of thing. Just remind him that it is healthier and safer for the animal if they are gelded.
If you had a stallion that you could not handle, could you really feel right by selling the stallion to someone else, not knowing if this person can handle the horse or not? It would make you feel bad if that stallion you sold to someone ended up hurting them.