Why does the dog need to be "fixed"?

So we got a new puppy. It is a girl, she is very cute. My son Ethan picked her out for his 8th birthday. She is going on 6 months and is going to be spayed in a week of so. Ethan had lots of questions about her surgery... Which brought about more questions about where puppies come from. I sweated for a bit and then decided I would just explain... He easily accepted all my answers and was fine with the idea of her getting her reproductive organs removed.... Then he asked about our 10 year old male golden retriever and asked how he got fixed. I explained and then looked at him....He looked like he was gonna pass out. Then he told me that was enough, he didn't need to hear anymore. I thought it was interesting .....and kinda funny.javascript:void(0)