My Dog Has A Yellowish Discharge From His Penis, It Smells Bad And He Is Fixed. What's Wrong?

I have three spayed male dogs who are with me all the time, and have never witnessed either of the symptoms you describe. It may be that they are all just extremely fastidious about their hygiene, but I think it's more likely that your dog has some kind of an infection or irritation going on.  
I strongly urge you to at least speak to a vet about it.

As long as it is emitting foul smell out of it, there is no need to get worried about the situation. This is just a normal discharge that is smegma having dead epithelial cells. It comes out of Mucoid Glycoproteins and many dogs go through it in their lives. If there is inflammation as well or itching is there then you should take it to vet for checkup.