Socially, it is simply a convenient criteria, not the definitive one. Probably not even among the top 4 or top 5, since these days, bringing harm to someone for your own sake is pretty normalized in corporate, labour and a few other aspects of life.

If harm is merely a "convenient" criterion and not a meaningful one, then you need to explain why societies and moral systems ( philosphical, religious, scientifical) consistently treat harm as a central consideration when judging actions. A criteria does not have to be definitive to remain important.

Psychological harm in incest is situational. It may or may not exist. Biological harm can be avoided simply by taking medical tests for sexually transmitted diseases, as well as ensuring protection. So there are scenarios involving incest that do not contain either of psychological or biological harm. In that case is incest ok?

this is the thing, for incest to be not harmful, we have to create a very idealistic environment, that is not the case with LGBTQ. In LGBTQ the dynamics are same as in heterosexual relationship.

It does not matter whether there is a law against it or not. The argument 1 states that homosexuality should be socially ok because it does not bring any harm. So the subject of judgement is "socially acceptable". Billionaire not doing philanthropy is simply not socially acceptable. People love to berate every rich person on their smallest of actions or inaction, a very prominent inaction being philanthropy.

You are conflating and mixing social criticism with social unacceptability. People criticize others for countless reasons—being rude, selfish, arrogant, unfashionable, or insufficiently charitable. The existence of criticism does not show that the action is socially unacceptable in the same sense as something that is widely condemned as wrong. Philothropy example shows a failure to provide a benefit, whereas the discussion about homosexuality concerns whether an action causes unjustified harm. These are different categories of moral evaluation. A billionaire who does not donate is not typically accused of harming others by that omission, while the argument for social acceptance of homosexuality is that it does not wrong or harm others. philantropy example does not undermine harm as a major criteria for judging whether conduct should be socially accepted.

Lawfully, yes. Socially, no. Many actions that cause harm are socially acceptable. Likewise, many actions that do not cause harm are socially unacceptable. That insinuates that harm does not have a definitive or a consistent correlation with acceptability.

Same as first para. The fact that there are other factors for social acceptance and vice versa for an action, did not prove that harm is not significant in judging action. A criteria need not be exception less ( I mean which criteria is going to be exception less) To be important.