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 didn’t 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)