The line of logic I always hear is:
-birth control is invented
-women are able to have sex with significantly reduced risk of conception
-more sex occurs
-increased occasionas of sex makes the risk of conception increase and close in on the pre-BC risk
-because people are now used to riskless incidents of sex, they feel entitled to no risk.
-Men feel more entitled to leave the women they impregnate, and women feel more entitled to kill/abort their baby/fetus.
-This increases single-motherhood which increases people being raised incompletely due to the lack of a father figure.
-This compounds more and more down the generations, as the first generation of incomplete men and women, produce another generation raised even more incompletely.
-Being incomplete makes a person significantly unlikely to be heavily invested in the well-being of their society. And so, they are more likely to work against the society's well-being, and therefore against the well-being of everyone else in the society.
Hopefully I didn't miss anything major.