is considered moral decline is a essentially a "thought crime."

We, as LDS individuals, IMO, get way too wrapped up in a black and white world view and are too quick to judge. If your (and it is basically is the Church's argument, which I find unconvincing, tbh) opinion that we are in a "moral decline" because worldly individuals believe having sex between consenting adults is acceptable, then that's fine. I just don't believe that to be the case. Nothing stops you from being a wonderful, moral person just because you're having consensual sex with your significant other.

Secondly, the decline in marriage rates has been pointed as not some sort of moral decline or a destruction of the institution of marriage, but rather due to socioeconomic factors (quotes / article below). Additionally, millennial marriages are STRONGER than previous generations (refuting the idea that people are morally bankrupt)

"According to a recent analysis by University of Maryland sociology professor Phillip Cohen, the overall divorce rate in the U.S. declined by 18 percent between 2008 and 2016. Even after controlling for factors such as age and other demographic shifts, Cohen still found an 8 percent decline in the divorce rate"

"A recent Pew Research Center analysis of Census Bureau data adds further color to the emerging image of a more stable, but more exclusive, family life. “Among all stay-at-home moms and dads,” Pew found, “those who are home to care for family are more likely to have a college degree than stay-at-home parents who are home for other reasons"