You raise an interesting question or rather a series of interesting questions. To avoid delving into the realm of pure opinion, I'm going to first enunciate the questions you are asking and then address these from Kantian and utilitarian/consequentialist perspectives.
I take your first interesting question to be this:
I. Does consent matter for consuming animals as food?
For Kantians, the answer in terms of animals is no. For utilitarians, the answer for a classic utilitarian would also be no. For the utilitarian, it might, however, still be wrong insofar as it increases suffering. For consequentialists, there is a type of consequentialist who wants to maximize consent in which case this would be more moral than consumption of meat -- assuming the animal can engage in consent. [which we will need to consider the next question to evaluate]
II. Does consent matter for consuming rational beings as food?
This modification changes the answer for the Kantian. Kantian ethics specifies in a formula of the Categorical Imperative that we must treat humanity [which here means rational nature] as an end and never merely as a means. Clearly, consuming a rational being to sustain our bodies is treating them as a means. At the same time, we are told that they are consenting to this.
But this won't do for Kantians. A good explanation of why is if we look at Kant's treatment of sex in the Metaphysical Principles of Virtue. Kant has a problem -- sex as he sees it is always the use of a person. Thus, for Kant, this has to be remedied by rationally agree to a marriage as a type of exchange of use rights. In other words, sex is use as means and has to be legitimated by a rational choice outside of the use as means moment. (Contemporary Kantians disagree -- see Christine Korsgaard Creating the Kingdom of Ends and Denis, L. (2001). "From Friendship to Marriage: Revising Kant." Philosophy and Phenomenological, 1-28.). But Kant doesn't think you can consent to the elimination of your rational nature. Thus, slavery and suicide are wrong for Kant (MPV). In other words, a rational being cannot consent to be the food of other rational beings since this would be allowing itself to be a mere means in a way that cannot be redeemed.
For Millean utilitarians, the rational animal difference doesn't matter. Or at best it would matter insofar as the harm principle applies only to such beings on some interpretations. To wit, how does the rationality of the food change the calculation if rational and not so rational animals can experience pain and pleasure? For consequentialists, it might matter if what is to be maximized is something related to rationality. Conversely, it might be more legitimate, if consent is the end-all unit we are maximizing.
III. Does the situation change from the above for eating one's own species?
I would say that there are not specific Kantian or utilitarian grounds for thinking so -- at least as they relate to consent and rationality. But you could be a consequentialist who wants to maximize something linked to being human in which case it would.
There may be other grounds for opposing this available to the Kantian -- or the Millean. Specifically, I'm thinking here of biological problems related to diseases when consuming your own species [their pathogens are ours -- their radical prions are ours, etc.]