Philosopher: It is absurd to argue that people are morally obligated to act in a certain way simply because not acting in that way would be unnatural.
Conclusion
It's absurd to say that people are morally obligated to act in way X, because acting in way not-X would be unnatural.
Evidence
An unnatural action is either a violation of the laws of nature, or a statistical anomaly.
You can't act in a way that violates the laws of nature, and the fact that something isn't usually done isn't a good reason not to do it.
Evaluate
There aren't any keywords indicating the conclusion, so we need to mainly identify it from the fact that the first sentence has the opinion of a Rebuttal. Since our author is saying, "people claiming X are being absurd", she will need to defend that position, and the rest of the paragraph tries to defend that position.
Since this argument is pretty hard to understand, it might help to come up with an example of something where "acting that way would be unnatural".
Maybe we could say . According to this position the philosopher is fighting, we are morally obligated to avoid giving our Dad flowers on Mother's Day because doing so would be unnatural.
That does sound absurd!
The philosopher digs in to this concept of "unnatural". She's like, which sense of "unnatural" do we mean:
1. unnatural, like transcending natural law? outside the realm of physics? or 2. unnatural, like 'weird, not customary'?
Her final sentence is exploring both of those options and saying, if we mean #1, then the original position is saying, . The author is like,
if we mean #2, then the original position is saying, And the author is like,
Goal
How would we describe the argument in general terms?
The author sort of creates an either / or (does "unnatural" mean impossible or unusual) and then follows both roads to say, .
We might seen language like the author considers two options and rules them out. Or we might see something like, .