it exist in only a few cases and is nothing compared to the ,,normal,, behavior
www.vin.com
like here or many other sides say - there can be other sexual orientated behavior to humans or other animals but it is not seen as normal for the species (doesnt mean its bad - only not normal)
some stuff is maybe conditioned (even unwanted) and a very few % of female dogs really wanna get breed but thats like sayed before the least part of all the female dogs in human ownership
many zoos tend to think they do good and all normal humans suppress but that is simply wrong - u can have a loving relationship to a dog without sexual content