[Agrippa]

Its not immoral, but the question should be, "should it be considered by the society immoral as a norm standard or not".

Because moral is part of our culture and should be useful in a certain sense like culture in general.
The human repertoire of behaviour was made for small group hunter-gatherers, it worked for that time and later cultural evolution worked partly quite good as well.
The human species is a species with an insecure instinct, thats why we developed culture, to bridge and finally outflank this weakness - so culture substituted our instinctive behaviour at least partially.

Culture has a goal, it has to bring in rules and norms to overcome the instinctive insecurity. Thats a creative process, but like the biological evolution, the cultural evolution can lead to a dead end.
The liberal society is a dead end and has to be reformed or if thats not possible destroyed.

If its about marriage, it makes sense from the group perspective, the question is how rigid should the rules be which decide about which sexual behaviour, especially of females, is acceptable for the moral of the group or not.

One thing we should clear up first if its about that is, should people live monogamous in all cases and should a marriage really last the whole life.

I think a compromise is the right decision, premarital sex especially of women shouldnt be accepted in the same way like its the case in our "modern" liberal-individualistic society of today, since we see to what this lead.
To an inflationary free sexual contact which corrupts both males and females and doesnt lead to stable partnerships or enough offspring.
Not to speak about the problems of stable social structures and the raising of children in a fatherless society.

Females dont believe in fix relationships in the same way they did anymore, at least if they made their various "experiences" and lost their belief in males or family, and most important, they dont have enough children. Males see sex just as fun and women as sexual objects in many cases, and no longer as their partner and future mother of their children.
But of course, thats a problem of our mobile, ephemeral, career-, job-oriented and hedomatic (hedonistic-materialistic) lifestyle in general and couldnt be stopped by living more virginal alone anyway...

Usually marriage should secure certain things. It produces partners for life, not just for sex, at least thats how it should be. It secures the woman if she's becoming pregnant and gives her help both from her husband and his family. The marriage should make sure that the woman will stay with the man and at least partially under the rule of the man and getting HIS children.
In most of the cases the spouse were not just a sexual partnership but a production unit (farmers, craftsmen etc.) in the past, which is much more stable than a partnership solely based on emotions and in the worst case of being enamoured, which is a feeling which usually doesnt last forever and is not the same as true love for a person.

That was lost during modernity and is of course a huge problem, especially in our times like explained above, because of all the factors involved.