"I caught it bad" is referring to how quickly and badly he fell in love with his secret partner. Lil Nas X has not seen anyone (romantically) for a long time - probably because of COVID-19.
"White" stands for cocaine. Lil Nas X does not support the hedonistic, stoned, partying lifestyle many successfull people have.
This individual leads a "dark" lifestlyle.
It to be that Lil Na X' secret partner is in a relationship with a woman. In Christianity, homosexuality is traditionally believed to be a sin, because God created Adam and Eve as a heterosexual couple.
Lil Nas X suggests to "commit adultery" (another sin) when his girlfriend isn't at home.
"Call Me By Your Name" (2007) by André Aciman is a romantic novel about two male lovers in a secret love affair. The story was famously adapted for the screen in 2017.
Lil Nas X finally embraces himself (as well as his sexuality and origins) by mentioning his own first name "Montero", which he wants his lover to go by as an act of intimacy.
"Montero" is also the name of a fictional land, similar to the garden of Eden, where the corresponding music video is set.
Lil Nas X becomes desperate. If his partner wants to have a certain thing, Lil Nas X wants to give that to him (or even become it).
The description of homosexual intercourse is still widely taboo - "shoot a child in your mouth while i'm riding" portrays gay sexuality in the same way that straight intimacy is often depicted in pop culture.
The track "Old Town Road" was on top of the charts for 19 weeks. Lil Nas X' fame makes him proud and changes his behaviour.
His openly gay identitiy is seen by some as a sign of "the world falling apart".
Lil Nas X may be referring to a straight man who he is lusting after - someone out of his (gay) league.
The seven deadly sins include lust and envy - Lil Nas X is lusting after a man with better qualities than him.
Their affair stays a secret but Lil Nas X is willing to keep it this way.