Bhagavad Gita

ಕುಲಕ್ಷಯೇ ಪ್ರಣಶ್ಯಂತಿ ಕುಲಧರ್ಮಾಃ ಸನಾತನಾಃ ।
ಧರ್ಮೇ ನಷ್ಟೇ ಕುಲಂ ಕೃತ್ಸ್ನಮಧರ್ಮೋsಭಿಭವತ್ಯುತ ॥೪೦॥
kulakṣaye praṇaśyaṃti kuladharmāḥ sanātanāḥ |
dharme naṣṭe kulaṃ kṛtsnamadharmosbhibhavatyuta ||40||

Sloka 1:40
Gist of the sloka:
When the clan/society sinks, the age-old dharma of the society is destroyed. Once the dharma is destroyed only evil percolates into the everyday behavior and society.
Arjuna, is indicating that once the clan/society is destroyed, the acceptable rules and regulations governing the society/clan is also destroyed as people who follow and enforce such moral rules are no longer around. Society collapses and entire society is now blanketed with evil forces. By continuing into the war we are encouraging evil and gifting it to the weakened society.
There is various dharma’s under which we go about every day: Jathi dharma, kula dharma, marriage/vivaha dharma, individual dharma and Sanathana dharma. We need to first understand them in little more detail.
Typically, we assume jathi dharma represents caste dharma – representing the 4 categories in the society Brahmin, Kshatriya, Vyshya, Shudra. Kula [samaja] dharma representing the trade or profession that one belongs to – priest, officer of the court, potter, farmer etc., Vivaha dharma indicates the various rites for performance of a marriage which again changes according to social strata, caste, time etc., While kula dharma represents the dharma for the clan/group of people, individual dharma is specific to each individual representing his beliefs, value and ethics. All of these changes over time, regions, applicability etc.,
Among all of the above, the Lord given dharma as mentioned in the Vedas is Sanathana dharma, which is never changing universal rules and regulations governing across all people, across all age groups and periods of time. Every other dharma must be in alignment with this dharma.
Our atma/soul characteristics is true nature and called ‘swabhava’. Unfortunately, due to family, clan we are born into, societal influences, how we grew up etc., this original character of the soul is covered with various layers called “prabhava”/influencers. This discovery of our own natural ‘swabhava’ character is very difficult due the various layers it is under. We therefore go with ‘prabhava’ believing it to be ‘swabhava’. Behavior according this ‘prabhava’ is jathi dharma.
Samaja [society] dharma: We are unable to live alone. We are product of the society and need the society to survive. As such, there are acceptable practices that one is expected to follow, to live in a society. Vivaha dharma is a subset of this.
For example, one who by nature speaks loudly is expected not to do so when he enters a quite zone like temple or hospital. He naturally lets go of his individual dharma [speaking loudly] and accepts the supremacy of Samaja dharma. Similar is the case with smoking etc., Basically, the society dharma takes supremacy over individual or other subset of dharmas.
This was the case with Vivaha[marriage] dharma also. Originally during the vedic period, girls were not sent to the gurukula. Rather they would be at a tender age of 8-9 years would be married to a gurukula graduate [around 16-18 yrs. old] boy. The boy would then start his own gurukula in the village where the girl would be his student for numbers of years until she matures to be a woman. By this time, she would know the husband very well along with his parental family, likes dislikes etc. and vice versa, She would also recognize him to be her first guru and respect would continue from both sides given the deep understanding already established.
In fact, both in Ramayana and Mahabharatha this practice of child marriage did not exist as by then society had changed a bit.
The child marriage again came into force during and after the various invasions from other cultures and religions. This forced the parents to re-introduce the system to safe guard the girl from being taken advantage of. Again, in the last few decades given the improved security the child marriage practice is being discontinued. In short, vivaha dharma is a societal offshoot and changeable according to the existing social conditions.
Sanathana dharma, is never changing, ever applicable, in all countries and societies and in all situations to all the populace and is cast in stone. For example, telling truth in all conditions, never steal, one should behave with integrity, belief in the universal lord are such dharma.
All other dharmas should be in alignment with Sanathana dharma. An individual dharma should be subordinate to samaja dharma etc.,
On the corollary, what means dharma could mean adharma [against] depending upon the intent and action. Eye is meant to see. It could also be used to see unwanted things. Same with other sense organs including mind, intellect etc.,
One should therefore use their intellect to analyse the given situation to see what is in alignment with dharma and what is not and appropriately act.