Sex is used as a scapegoat for many problems inside and outside relationships. It is used and abused even when other means are called for.
From saving relationships to cheating, here are some of the things sex doesn't do.
It is/leads to intimacy
Sex is a form of physical intimacy but not intimacy in its entirety. Physical intimacy includes other things like touch, hugs, kissing, playing, eye contact, and talking. Another aspect of intimacy is emotional intimacy. Although sex can include both, it is not the only form of intimacy. In fact, it is a result of intimacy and rarely the cause.
Just because you have sexual chemistry and attraction doesn't mean you have intimacy.
It fixes relationship problems
Unless sex is the problem, it can not fix other relationship problems. Relationship conflict that is not rooted in sex cannot be solved by sex. It can give relief from the conflict but it is not therapy or an effective communication style to resolve conflict.
Can save a relationship
Breakup sex is a popular term, but along with sleeping with an ex to get back together and "improving your game" to keep a relationship, it doesn't save relationships. Whatever problems a couple has have to be addressed directly and not under the guise of sexual intimacy.
It is supposed to get better
The sparks and chemistry in new relationships can set the standard for sex. But it is not realistic to expect it to be that way. This doesn't mean there's a glass ceiling on the quality of sex, but other relationship qualities come in to determine. Connection, compatibility, and communication are key to preserving a thriving sex life.
Lack of it or bad sex leads to cheating
A lack of communication and an absence of nurturing the relationship bond leads to cheating. If partners are on the same page about their needs and work to find common ground, then cheating is unlikely to happen. So saying that the lack of sex and poor quality leads to cheating is an excuse and a terrible one at that.