ETA: Actually Souther girl since you said "obviously" it changes why does is "obviously" change?? This is a sincere question because I don't understand why it changes and so I especially don't understand why it is obvious!!
I really don't have time to do the reply justice but just look at this: there was a famous study where an attractive woman propositioned random strangers for sex, and a sizable percentage of guys (I think it was 75%) said yes. These were young men, if I recall (undergraduates), but you see the trend. None of the women propositioned by an attractive guy said yes.
In a committed relationship, several things change. On the base biological front, you'll have a hard time carrying that first endorphine rush more than two years into a relationship, so "just sex" becomes harder for both genders. Sex now "means" something in the context of that relationship, both positive and negative day-to-day stuff can influence both the man's and the woman's willingness to have sex. It can be an expression of love, tenderness, anger, a tool for manipulation, a part of an intracouple power struggle - the list is endless. There's great potential for committed couple sex to be far better and far worse than "just sex" with a casual aquaintance.
