Paula Hall, a UK Sexologist, say sex follows the 2-6-2 rule. For every ten times you have sex, two will be great, six will be okay, and two will be boring. I have three thoughts on this.
First: I think there is a lot of truth in this. The percentages will be different for different couples, and the range of experience might be wider, narrower, or more to one side of the scale Hall gives, but in general sex is variable.
Second: When you have sex infrequently, those “bad” times seem far worse. If you are going to have sex again in a day or two, the boring time is no big deal. If that’s it for the next week, or ten days, it feels far worse.
Third: Practice makes perfect. The more you have sex, the better your odds of sex on the great end of the scale, and the better great will be.