This sentence makes no sense. It's likely trying to say that you should not have an affair but the English is poor and sex is not a possession that you can steal.

https://books.google.it/books?id=vGuRBQAAQBAJ&pg=RA2-PA20&lpg=RA2-PA20&dq=%22This+means+that+a+married+person+should+not+steal+sex+from+a+person,+married+to+another%22&source=bl&ots=fBzD8Pxl-l&sig=f6AkvjE10Yr-smKhmEgCh1MgfBY&hl=it&sa=X&redir_esc=y#v=onepage&q=%22This%20means%20that%20a%20married%20person%20should%20not%20steal%20sex%20from%20a%20person%2C%20married%20to%20another%22&f=false Interesting, it makes sense, if you commit adultery, you are taking sex away from another relationship. It's not always a zero sum game, but I can see a situation where it works like this.

This form of the word steal is like to say 'steal a kiss' so yes the mean adultery or affair but it is poor English, because while a kiss is not a tangible object it is a thing, thus it can be used as a noun just like emotions are not tangible objects but are nouns that are not people nor places thus things so can be treated as a noun, but while sex can technically be used as a noun, it is never use this way. Instead we convert it by saying a sexual encounter or something like that so you could say one should not steal a sexual encounter with... it still sounds awkward. It seems to be one of two of the Christian Ten Commandments either thou shalt not commit adultery or thou shalt not covet thy neighbor's wife.