I have an intact mini schnauzer just over 18 months old - he seems very attractive to other male large dogs which have been castrated.  German Shepherds and large black retriever type dogs drool around him and then try to mount him. As you can imagine, he hates it and is very often pinned down beneath the weight and I have to rescue him.  He is a really placid little thing so he doesn't snarl - just goes really submissive and tries to get his bottom out of the way.  This probably isn't unusual but I was curious whether the castrated dogs lose their natural instinct to properly differentiate between male and female.  Also, I attempt to push the larger dog away but I realise that this might not be the wisest thing to do,  If I call Oscar to me, the larger dog just runs after him and he looks even more vulnerable with his back to them.  Any advice on how to handle the situation. 

I would ask the owners to not allow their dogs to do this, I have heard of fixed dogs having this problem. maybe to if your dog is alot more submissive, or the other dogs are just playing. I have had my fixed ones playing with other fixed ones and they all mounted eachother, it was a female that was mounting my male. Im sure someone will be along with better advice.

This mounting is not always, (hardly ever, in fact) to do with sex. It is one dog showing the other that he's way cooler, much more hip and in the groove, stronger in every way and he wants to be looked up to and revered by the one who drops to his back. (that's kind of in human translation) LOL. No, seriously...it's kind of showing that "I'm dominate. I'm stronger, so until you go on your back and acknowledge this fact, we aren't going to be bro's." Young dogs do this in play. It's practice for when life gets serious. I wouldn't worry too much unless one dog is way heavier and can hurt the other. It's good if they hurry up and do it and get it over with so they can establish who's who. But then there's nothing wrong with human mom saying, "Ok, ok...enough is enough. You all know, I'm the coolest. I'm also the one with the treats"

Carrie

Well I am certainly not an expert on this but the whole performance starts with the larger dog sniffing intensely around the privates of my dog.  My gut feel is that it is nothing to do with dominance, as I said in my initial post, they literally drool over him.  If Oscar sits down, they proceed to try and sniff him as best they can.  I just don't think it is dominance.  Some dogs have been grabbed by their owners and walked in the opposite direction until we are almost out of sight.  They let them off and their dogs come galloping all the way back to him completly ignoring their owners.  On one occasion I was so concerned that I picked Oscar up when I saw the German Shepherd hurtling towards us - and then the GS just sat looking at me licking his lips :(   All the owners without exception are extremely apologetic and it only seems to happen with the larger dogs.  I always have to tackle the situation because they are probably 4/5 times the size of him.   Sounds like we just have to live with it.