That's 2 different scenarios.
Ultimately, sex is part of a relationship for most people; i don't know if I'd use the word 'necessary', but it's anticipated it will happen as an expression of love.
However, it's perfectly normal not to want to get involved with someone who just wants sex. You do you, and if you want to wait until you have more understanding of and are comfortable with someone to have sex, that's fine. Don't get pressure into anything or do something just because everyone else is, or says they are!