Sex without marriage is not a sin, however marriage without sex is a sin!
No, marriage without sex is not sin.
A sexless marriage is a marital union in which little or no sexual activity occurs between the two spouses. The US National Health and Social Life Survey in 1992 found that 2% of the married respondents (aged 18 to 59) reported no sexual intimacy in the past year.
Sexless marriages can develop over time from a range of possible causes.
Some couples may have sexless marriages because they have different work schedules or busy lives.
Adultery can lead to a sexless marriage, for "a low level of sexual desire" due to age.
Difficulty during any stage of the sexual act.
A marriage may also be sexless if one or both partners is asexual or if the couple mutually agrees to abstain from sex due to the goal of avoiding conception.
Some couples may be married solely for legal purposes or tax benefits.
https://en.wikipedia.org/wiki/Sexless_marriage#Causes