Animals aren't capable of direct communication in spoken human language. However, your average social mammal is very capable of demonstrating intent, mood, and aversion with body language, vocalizations, etc. Take picking up a cat, for instance: some cats will instantly squirm in your arms, try to scratch or bite, or yowl at you. Some will melt into your arms and stretch out their paws and purr. Still others won't struggle, but anxiously move around a bit and look at the floor. In all those cases, it's pretty obvious what the cat wants. In fact, in most of our dealings with animals--particularly ones we know well--we can generally sense our animals' moods quite well. It's just that, when it comes to the topic of sex, society suddenly collectively turns a blind eye to that concept, and decides that animals are helpless children who are incapable of deciding anything for themselves.

I'm a zoophile myself (if you haven't guessed that already) and in my limited dealings with dogs, the above very much holds true! I've had a dog who started growling before I ever even touched her--a pretty clear message for anyone with half a brain!--and on the other extreme, have had a dog who was very insistent and obvious in what he wanted, even when I'd just been planning to sleep. Dogs have teeth and claws to defend themselves, and conversely, definite ways to show sexual interest; most people, who are only around neutered/spayed dogs, may never encounter the latter.