There are these things called amino acids. Those are very important chemicals that our body needs to synthesize lipids and proteins(the building blocks of our body).
Out of the hundreds of millions amino acids out there there are 20 amino acids which the human body cannot synthesize by itself. They are called essential amino acids.
The most convenient and abundant place to find all the essential amino acids is red meat.
Humans are genetically programmed to release hormones that make you "happy" when your body has everything it needs for it's well being. Ergo we are genetically programmed to like red meat a lot like women are attracted to men who have power, be it money, physical strength, intelligence, good look, etc., because they are searching for the male that can deliver the best genetic material, and protect her and their offspring. And like males are cheating by nature, because they are programmed to spread their seed as much as possible. It is deeply encoded in our DNA from our beloved mammal ancestors. It was the way to survive millions of years ago and essentially, no matter how far we have come in our development, we are still mammals. (let's do it like they do it on the Discovery channel, lol
This is why mcdonald's and burgerking are killing it. Lots of calories translates as "super delicious" to our humoral system. A lot like how smelling works. There aren't "bad" smells. There are just chemical substances that react with certain chemicals in our nasal system, which is transformed into certain electrical charges, which tickle certain neurons in our brain and we thing of a certain chemical substance smells "good" or "bad". It's just a complex electro-chemical reaction "grandfathered" from previous stages of our evolution.
Having said that, environment > genetics. And as we live in time of abundance (in the Western world, of course) you don't need that programming any more, because you are not facing the danger of not having what to eat in the next week or month. You don't need everything in one place. Mammals needed meat because of fat reserves and because they didn't exactly know what nutrition they needed. They just blindly guessed what is best(trial and error). We can deliver everything we need from other sources, rather from just one. And we can overcome the genetic programming and stop eating meat and be perfectly fine. From biochemistry's point of view, nothing will chance(if you really get EVERYTHING you need, which is not an easy job at all, like a lot of people are thinking). And you have the eastern way of eating where red meat is rarely consumed, and yet those Japanese folks are doing just fine, I hear.
That's all from scientific POV. I'm always up for a steak.
FWIW, I have 2 chemical educations, in general, and organic chemistry and can demonstrate everything with structural formulas, no one will understand, but that would be an overkill, I guess.
PS: Good point for the aliens though, have't thought of that ever.