I am not going to comment on pro-mortalism, as I know very little of it.

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Now, unfortunately I fear you have only been given or are only producing a strawman of both what anti-natalists is, and what it argues.

To define anti-natalism (AN): the belief that it is wrong to bring new people into existence. (This could extend to all life, if needed). It is not a position on what someone should do once they exist, in how to live a fulfilling life, except for the case of arguing a person should not have kids.

As to why: because there is suffering in existence - this is an important point, most arguments used by AN rely not upon the belief that existence is suffering, but that it has suffering within it. From this premise, we derive some of the following arguments:

• Axiological Asymmetry: Existent Benefit = Good; Existent Harm = Bad; Non-existent Benefit = Neutral; Non-existent Harm = Good. (Benefit and Harm here refer to pleasure/pain, knowledge/ignorance, esteem/esteem-lessness).

The allegory used here is: we do not think it is bad for the people who don’t exist on Mars to have no benefit, but do think it good that non-existent martians are not suffering because of such.

• Wellbeing Argument: Existence is majority or entirely suffering vs benefit - Benatar comments on this, supplying scientific studies showing that people’s memory tends to prioritise positive memories over negative ones, even in the case where their life has been relative hell. It is plausible people’s desire to survive is an evolutionary mechanism which increases the chances of reproduction; a person might be perfectly capable of living an unhappy life, incapable of understanding it as such, if their genes incline them towards ignorance. (I personally disagree with the wellbeing argument holistically, as a metaphysical reality of suffering, but I agree that some people’s lives are hell, that they are blind to such a fact, and, despite their circumstances not altering, these peoples still bring new humans into their damaged situation.)

• Probabilistic-Insecurity Argument: we cannot secure the beneficial, no-harm existence of a person we bring into existence, absolutely; hence we shouldn’t bring them into the world.

• Non-consent Argument: given anti-natalists believe, as well as any other sane person, that existence has suffering within it, bringing people into existence without there consent is regarded as wrong. For an analogy: you run a bath; some of it is boiling, hot, tepid, luke warm, cold, or just right. Without their permission, you throw a person into the gigantic bath, without knowing if they will burn, freeze or relax; this we regard as wrong.

• Damnation Argument: this is only reserved for Abrahamic religions but relies upon two points. Firstly, abrahamic religions accept the premise that our current existence is suffering, and only some form of divine act can save us. If they are wrong, about God’s existence, this still leaves existence to be suffering, hence we shouldn’t bring people into the world. They also believe that if man fails to have a relationship with God, atone for his sins, and submit to God, they may be damned into the endless pain of hell. Given a parent cannot ensure their child’s salvation, it seems irresponsible to possibly doom them to eternal suffering.

There are some other interrelated but non anti-natalist arguments:

• Non-natalism argument: instead of it being an injustice to bring people into the world, there simply isn’t a justification for bring new people into existence.

• Environmental arguments: more people will destroy the world quicker.

• Adoption argument: it is better to adopt the millions of kids without parents.

• Vegan Arguments: less people, less animal food produce.

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As to why we shouldn’t build an utopia: well, I don’t totally disagree with you. I would personally hold that the hedonistic imperative is an obvious conclusion of anti-natalism, when one accepts that humanity will never be wholly anti-natalist. However, the initial imperative to not have kids still is primary.

Despite this, it does not follow we should accept suffering now for the benefit of future generations. An example of this is the classic: ‘maybe I should have kids because they could be the doctor which will cure cancer or a scientist which fixes climate change?’ - however, it does not follow we should subject another being to suffering to resolve our mistakes.

The important point is that anti-natalism and hedonistic imperitivism are not mutually exclusive positions, as long as the latter does not hold that bringing new people into existence is a predicate for the achievement of their vision. We have no right bringing people into existence to achieve our dreams and desire.

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I also want to point out that reddit anti-natalists are usually not philosophical anti-natalists. They are often ill-informed 20y old whiners with poor life prospects and too must screen time. If you are engaging with reddit anti-natalists you are probably debating the equivalent of a high-school feminist with daddy problems or neo-conservative with mommy problems; not the serious feminists, conservatives or anti-natalist of philosophy and political studies.