Sex in the early days
When you meet someone and fall in love, your whole life revolves around getting to know them better, particularly their body. After a while, however, you realise love won't pay the bills and you settle down to 'normal' life.
This is generally when sex becomes something you do at night in bed - preferably before you fall asleep. But, after a hard day's graft, sometimes there just isn't enough energy left.
Quality not quantity
At this stage, quality becomes more important than quantity. When you're having sex as often as you like, it doesn't really matter if you have the odd unsatisfactory encounter. But if you're only managing it once a week - if you're lucky - you need make the most of it. Which means making sure you're not hanging on to any unrealistic expectations.