It is not just the poo, it is the fact that dogs are allowed to pretty much roam free in most of our urban green spaces and like other posters have mentioned, i'd like to be able to sit down for a picnic or play with my child without a big dog bounding over and scaring her, she's only 2, many dogs are much bigger than her which must be scary - although i always tell her it's ok, the doggy is just saying hello, do i try and stop her being scared. I'm not bothered by dogs, but i think they smell, they slobber on me (and eat their own shit) and are hairy so i don't really like to be around them much, but in the park we have very little other option. It should be the other way round.
I/we can't do much about other animals, ie birds, mice, foxes they are part of the rural and urban landscape and are more than welcome to go to the toilet where they want. We can do something about dogs being all over our green spaces, humans choose to have them as pets and where to let them go to the toilet and run around, we can restrict this in public spaces so we should - i'm not saying all, it would just be nice for humans to have a bit more priority.
There were over 10 million dogs in this country in 2011, that is 1/6th of our population - far to many imho, i like what a previous poster said, no garden, no dog.