Personally, I'm a romantic guy, I'm quite intimate and therefore, I like sex.
I think it really depends on each individual requirements, some people may not like having sex often, or simply just don't have the time for it (if they have a busy life), but I think sex is a big part of making a relationship work, as it maintains intimacy and I believe over time, it enhances your love for that other individual. I believe theirs some science behind the scenes, that when we have sex with another human, possibly, we get more attached - who knows, but survey results have revealed that married couples believe that sex is an important aspect.
Like I said, it depends on your requirements, if you're having sex with your partner one a month and you're happy with that, then what's the fuss? Problems may arise if one is satisfied but the other isn't, so, I think good communication is essential for any relationship to work.
If one is happy and the other isn't, that's when cheating may occur, then the innocent half of the relationship will start to question why it didn't work, and eventually, they'll work out that maybe the sex wasn't as great as they thought.
Of course, sex isn't everything, but I think it's important!
It's also important to note that it's normal for sexual activity to decrease as the relationship moves on, it's at its peak when you first get together into a relationship!