The guy is desperate to have a GF that is why he stays with her, from her side I don´t know maybe using him for money while fucking other guys on the side?
But seriously if you´re in a relationship and don´t have sex your that is a clear sign that something is wrong, either she can´t have sex because of physical problems (VERY unlikely), or she is not attracted to you which would be a reason to fuck other guys on the side.
But age is also important if we are talking about tweens then it´s obvious why they haven´t had sex but for teenagers and adults then there is something wrong and it´s 100% the female who are refusing to have sex because men never would turn down sex to an attractive girl.