Sex in a relationship requires work after the honeymoon period is over. Girls give it out on tap in the beginning with little or no stimulation so the guys dont understand why all of a sudden things have changed...but they do
It becomes like eating the same meal everyday for a week or two, same basic meal with no sauces or spice. But it takes work on both sides to introduce new ways and this is where you need to start talking, it's an issue that needs your attention because sex is important in a relationship, sometimes more important for one than the other & you've already recognised there is a problem.
It happens in almost every realationship, it's how you deal with it that matters LR.