Why is it these discussions always get hijacked by the same old rants from the same old people?

It appears to me that societies evolve a moral structure which reflect's those societies' needs. They then use religion as an effective means of enforcing them. However different societies have different needs, and what is considered moral behaviour differs between them.

For example, farming societies are very interested in inheritance, to pass the land on through the family. It was in those societies' interests to encourage sexually faithful family relationships to protect inheritance rights, although opinion varied whether these should be monogamous or polygamous.

The South Sea Islands, which were small geographically isolated populations, faced different problems - inbreeding. For their societies, the most moral thing its women could do for the benefit of the community was to get pregnant by any passing males who sailed by, to add to the gene pool.

In the UK, and I suspect the US, where sexual activity has become separated from pregnancy and where women are no longer financially dependent on men, society has evolved a new sexual morality. Many of the old reasons for restricting sexual behaviour no longer apply, but new ones have arisen. Virginity is no longer prized. People are expected to be monogamous within a relationship, but may have multiple serial relationships. For those not in a relationship, sexual abstinence is no longer expected, but practicing safe sex is.

You may not approve, you may not consider it 'moral', but if morality is a shared code of behaviour then that what it is.