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Because animals cannot give consent and there are laws protecting animals from abuse.

The whole issue of "consenting adults" is always played as a "trump card" to cut off discussion. But consider:

If someone wants to get their jollies from a melon or a cucumber, no one wonders if the vegetable is capable of "giving consent." Consent isn't an issue. Even if the vegetable dies from the procedure.
When I take my cat to the vet, I don't concern myself about whether or not the cat can "give consent." I doubt that the cat would consent to getting his shots. Nobody seems to have a problem with that.
The cat has been confined to our house for nearly a year, even though he has shown clear indications of wanting to go outside. The issue of "consent" never arises. Why is that different?
If a dog starts to "mount" someone's leg, and they don't stop the dog, but both enjoy the experience (yes, I have heard of people like this), would this be "abuse"? Does "consent" matter here? What if the contact is more "intimate" but the experience is still mutually enjoyable?
A person can stipulate in their will that their body is to be used for scientific research. Inasmuch as consent is an issue, the will constitutes consent. I believe that parents can give consent in this fashion for the body of their child. What if parents determined to sell the body of their child to a pedo-necrophile? Is anyone getting hurt here? Is any "force or fraud" involved? Is anyone being abused?

Granted, the last couple of examples are...utterly revolting. But from a utilitarian viewpoint you have no particular reason to make laws against any of these. If you were going to outlaw dealings with animals based on "lack of consent" and "abuse" you would probably, like PETA, start with nonconsensual confinement, treatment of food animals, ownership, etc. and sexual contact that the animal does not enjoy. But why should we single out only the last of those? And why should we care what a pervert would do with a corpse he had acquired legitimately?

I would suggest that there are acts--such as some of these--that are generally detrimental to society as a whole. And that it is good if there are laws to punish some of them. And finally, that "consenting adults" should not be considered to be a magic bullet for killing all morality arguments.