Why, for instance, is murder deemed wrong in society?
If a person were to eliminate his competition in a bid to immediately put himself into a more advantageous position than his former competitor, murder presents initial benefits. But if he could do so freely, his competition could likely apply the same principle on him. 'Freedom to kill' poses a mutual risk to both of them. On a larger scale, it poses as a threat to everyone within a social community.
Outlawing murder is a development that is engineered to improve overall survivability. Because it is beneficial, it is thus made 'right'.
The question of lust can also be evaluated in the same regard.
The question of social discrimination could also be evaluated in the same regard.
Morality is a human construct. The only measure of rightness and wrongness is the consideration of motivations and consequences.
As you had proposed, plants have no capability to experience physical pain. Since a wildflower would feel no pain, would you happily crush any under your feet whenever you see one?
No you would not.
Not because it is right or wrong, but because it isn't necessary.
Imagine insects, birds and small animals that have to be killed or deprived by other means of a viable and plentiful food source so that the greens may reach your table fresh.
Imagine wild animals displaced from their natural habitats and their way of life because of expanding human settlements.
For the predator to live, is it not necessary for its prey to die?
For the prey to live, is it not necessary for the predator to be subjected to hunger and death?
In order to turn a profit, must one not sell to a buyer a product at a price higher than its worth?
In order for a company to offer you that one position, is it not necessary for other candidates to be turned away?
Your own survivability and prosperity hinges on benefiting at the expense, or even exploitation, of others.
Whether you are aware of it or not, what your feeling or not feeling changes nothing.
By nature, we are neither herbivores nor carnivores. We are omnivores.
This adaptability to a variety of diets has been the key to survival throughout our evolution.
Meat eating is neither right nor wrong. It is as necessary as the consuming of any other food source.
Having the luxury of being in a modern environment where choices are abound does make the struggle for survival seem that much less trivial to most. As a result people's perspectives are constantly clouded by misapprehensions and prejudices about the identity of being human.