If a person has the wherewithal to consent to things, what would the specific act(s) they consent to or don't consent to have to do with whether or not that person CAN or CANNOT consent?
Let's say your neighbor owns a dog and you consent to letting it chase a ball in your yard. We could say you then have the wherewithal to consent to actions pertaining to the dog and your property.
Now, let's say the dog starts humping your leg and you enthusiastically consent to that heinous act, how does the nature of your engaging in a consensual, yet heinous act with your neighbor's dog, change the fact that you possess the wherewithal to consent to the leg humping?
Now for the record, while I'm not in favor of you humping the leg of a 13 year old dog, I can see how you'd feel cheated if you didn't get your turn.