A 21-year-old woman has explained why she is going against the cultural grain and saving herself for marriage.

The woman writes anonymously in The Guardian that she made the decision at 18 to abstain from sex outside of marriage. 

And while she doesn't think sex before marriage would make her dirty or impure, she believes it is a sin.

'This is a decision I made at 18, because I believe that sex outside marriage is a sin,' she writes.

'I believe that marriage is what God has given us so that we can experience a glimpse of the love between Christ and his followers. Ultimately, when I sin, it's an act of separation from God.'

The Guardian's 'My Life in Sex' column invites a different reader each week to talk about their views or experiences of sex.

In the latest column for the series, the unnamed woman writes that she is 'not a saint' and describes herself as a 'normal uni student who has definitely had a few drunken kisses on nights out'.

What she really has a problem with, she writes, is the assumption among some men that they can just have sex with her if they want to.

'I've also had my head banged against a wall by a guy I was kissing because I didn't want to go any further, and once had to keep removing the hands of a guy from going down my jeans,' she reveals.

'I don't want to engage fully in a culture that treats sex so casually, and has resulted in some people feeling entitled to it.'

According to the British Sex Survey in 2014, the average Brit loses their virginity at the age of 18 but most (92 per cent) still think that monogamy is desirable in a relationship.