Keeping sexual communication open throughout your relationship is critical. As one partner starts closing off, and not just having periods of low sexual desire, but shutting down the idea of sex, something is going to break down.
Sex with someone, anyone, is not very much more fulfilling than porn and my own left hand. So I don't look for this type of sexual gratification as an initial part of my dating process. And I'm pretty clear on my dating profiles that I'm not looking to hook up or jump in the sack.