You are missing the point. There a countless things that are beneficial to humans that with inevitably mean harm for someone else. If two men love the same woman, and she only chooses one, how is the one that was not chosen not harmed?
If two people want the same business contract, and only one is chosen, does it benefit the fortunate contractor less?
You have to pick one.
By process of elimination, there has to be one person out of everyone else in the world who is best for you, because all people are UNIQUE. Because all people are unique, there is no way that two men could love any woman equally. They could love her approximately the same, but if that were true, then neither one of them should be with her, because she is not best for either one.
Of course, from the woman's point of view, if the two men are so very similar in attractiveness to her, that is another indication that they are BOTH the wrong man for her.
Maybe they are the most attractive men she has met SO FAR, and the woman is the most attractive woman that both men have met SO FAR, but that is far from any of them being the best match for each other.
Of course, there is the possibility that one of the men is a really good liar, especially to himself, and the woman is also cut off from having a good sense of her emotions, in which case the other man could indeed be the right man for her, but that is very rare. Two men cannot be the best choice for any one woman, and two women cannot be the best choice for one man. It is a logical impossibility because all people are unique. If ever you really are attracted to two women equally, they are BOTH wrong for you.
I said "if one is concerned with doing what is REALLY beneficial for oneself" . That means REALLY, ACTUALLY, ABSOLUTELY, in other words, the best. Not second best third best, 4th, 5th, 6th, 7th, 8th, or 30th best that most people settle for as they sell out their ultimate best interests for those that seem easier and more expedient for the sake of short term gratification, but BEST as in first best. REALLY in one's best interests. There was a reason why I capitalized the word before, which was to emphasize the importance of it being in reality, as in what really works, and the better something works, the more it really works.
Now you mention the business contract. Again, it's about best interests. If the person is acting in their best interests, the person will not be involved with a company that pollutes or upsets the ecology to the extent that it reduces the quality of the natural environment, which is the human life support system. If a person is truly acting in their best interests, they will not be in a business that has such a finite amount of opportunities, or rather, a reduction in business, the more business that they do, like old growth forest logging, without replanting trees, then the loggers complain about not being able to cut more because of the spotted owl, but the reality is that the owl is just a symptom of the fact that THEY already cut down too much already. Had they planted trees as they cut, they would not have that problem.
Doing what is in your best interests starts at the BEGINNING, otherwise, you cannot do it, because if you compromise at any point for the sake of some business or goal and are then in that position of having to defend it, you are already not doing what is REALLY beneficial for yourself. There are many businesses that are wrong, and by simply being in them, one is not doing what is in their best interests, or perhaps they got into a business that was already completely saturated with enough other companies that do they kind of business one wants to do, and so to get into that business in the first place is not really in one's best interests. You need to see the big picture in order to do what is in your best interests. I do not believe that competition is really in people's best interests, to make it more clear. I think that a creative orientation which is perfectly aligned with harmony is. That is what it really is to be human, because that is what human beings are capable of, and anything less, is less than human.
This last part you wrote: "Self interest is the same for all organisms. A lion gets his meal, and the gazelle loses his life. It benefits the lion no less, and the gazelle no more. The difference between humans and animals is that morality becomes an issue for the human, but you cannot use the benefit of the one
and the absence of harm to the other as simultaneous standards of whether the action is wrong or right. "
That is a perfect argument to defend all criminal behavior. Rapist vs victim, thief vs victim,or any time anyone gets some sort of benefit from taking advantage of another. One does not need to take advantage of other people in order to feel really fulfilled. This is civilization, which was made so people could develop art and science and knowledge without having to be concerned about dealing with aggressive vicious animals all of the time, NOT so people could use it as an opportunity to better take advantage of their fellow man or have boxing or UFC to appease people's regressive kind of animalistic instincts or tastes.