i think "sex industry" jobs are often a poor choice of career-- especially today-- with so many new and deadly diseases-- but the morality of it-- all lot of that is social-- and related to patriarchies--. As jewish law points out, a child of jewish mother is always jewish-- but a child of non- jewish mother and jewish father is not-- (i suspect the same held true when cultures, like india, had a "caste" systems-- or cultures where you get "born into" the society-- it is not true with all societies-- but many)

A lot of is about making sure the children your wife/spouse/partner bears are your children-- in days long before blood test or dna!

It is these cultures that create the Madonna/whore dicotomy-- they are fearful of "loose women"

in culture that have matriarchies-- where it doesn't really matter who your father is-- your family, status, inheritance is all through your mother-- prostitution is not much of an issue-- but its also much less of a choice-- since women tend to have "inherited" status-- and don't need a man to get status--or material assets!

I tend to think prostitution is exploitive-- it exist because men have "visible sign of power" (ie, money) and women have limited means of getting it-- They can use their appearance -- and trade it for a good marriage-- or they can use their families (male members of the family) to arrainge a marriage (the "groom" doesn't so much marry the bride, as he creates an alliance with the bride's family)-- but still, the women only has status as it relates to the men in her life-- and since they want to be "sure" the children are theirs-- they want to control a women sexual life-- and a woman with an uncontrolled sexual life is held in contempt.

So sexual purity becomes a value-- and sexual looseness a sin--or worse! In many cultures -- a woman can be killed for having sex outside of marriage- but the man who has sex with her is held blameless-- and even "modern" civilized cultures-- US or England-- there still remain an attitude that a woman did something "wrong" if she get raped-- and it is very hard to convict rapist-- a woman testimony isn't enough! (thank god for DNA testing-- it has become a "co-operating witness" to the crime!) and in the same societies-- prostitutes get arrested-- but "John's" go free.. two parties to the transaction, but only one is wrong. (do i see a double standard?)

There are some "sex industry" jobs-- that women enter into freely-- "exotic dancers" in many cities work as independant contractors-- and can earn very good incomes-- and prohibbit physical contact from "customers". but in most places-- prostitution is exploitive.. not morally wrong-- (but pimping is!) but a societal wrong--
It doesn't have to be exploitive--or wrong-- but it has become so...

An other point--she is called "Maid Marion"--Not Mrs (madam, mistress)-- which implies she is not married or "troth" to Robin-- so her actions are hard to define as "unfaithful". Should/does Robin have the right to deside what Marion gets to do with her body--just because he is a man?
If one gets married-- and make a promise to be "faithful"-- and "foreswears all others"-- okay, then she is breaking a vow-- but is she is Maid Marion? (the story is unclear as to whether Marion and Robin are just a "couple" or if they are "husband & wife"-- )