There could be lots of reason for not having sex:
Asexual
Low sex drive
Never found a person they could connect with like that
Being an awful person / falling into the internet incel category
Different motivations in life that they didnโt feel the need for sex
Medical problems - this could cover a lot from psychological issues to physical issues
Fear of pregnancy / failure of birth control
Strict religion
Nervous/anxious/shy/awkward personality (like in the film)