Jason. From your question or from your profile its not clear which part of the world you are from as it would have become easier if we would have given any idea about your background.
What you are thinking or wondering about is quite natural means yes it is true that having sexual relations with the person of opposite sex (or with the person of same gender too these days) is considered the basic necessity of the person . But in most societies of the world women are taught to supress their sexual feelings anyhow! In many cultures having sex is considered as sin , genitals are associated with dirty organs, watching sex or doing sex is considered as taboo! So in the minds of many women it is cultivated that sex being a dirty, nasty act which has to be done in dark with minimum possible festivities and quicker the better!
But then above things are NOT necessarily applicable to your mom as sha have enjoyed active sex life for better period of her youth. She just might have been thinking that she has gotton aged to enjoy sex ! And believe me friend for many of the female clan sex is not as important an issue or matter as it is with their male counterpart! Even if once in a while she might have been getting a feeling of having sex that you can not fathom ! I would suggest you to concentrate on your studies and career and stop about the sex life (existing or non existing) of your parents. They are older, matured and wise enough to think about it! You should be the last person on this planet to think anything about it!