ImO that POV is kind of shallow, sorry.
First, depending on the person there are (many) different reasons for them to marry someone. If a relationship is mainly based on sex, it probably won't last very long.
Second, things may change and most often do. People change, relationships change, and sexuality changes too.
A mid-40s wife may be in her sexual prime and attracted to different things or people than when she was 21. A husband's libido and/or "erectile skills" may decrease with age, which can create problems for the couple's direct sex life. But that does not necessarily mean they would not love each other anymore! - That was just an example.
There are many different possible reasons for a couple to decide to sexually open up their relationship (e.g. on the Woman's end).
In my personal life i know some people who have a clear preference for persons with a particuar physical trait - as far as sexual attraction is concerned - but "ended up" falling in love with and marrying a person that does not have those traits. There are so many different factors involved in the process of falling in love, having a partner and the decision to marry a person. just sayin'