Remember this scene from "Annie Hall"?
[Alvy and Annie are seeing their therapists at the same time on a split screen]
Alvy Singer's Therapist: How often do you sleep together?
Annie Hall's Therapist: Do you have sex often?
Alvy Singer: [lamenting] Hardly ever. Maybe three times a week.
Annie Hall: [annoyed] Constantly. I'd say three times a week.
You hear all the time about how married couples have sex three times a week. I always thought that sounded a bit high because there is a woman involved. (In the case of "Annie Hall", three times a week sounds high with a man involved who looks like Woody Allen).
Based on more than 11,000 GSS cases, I generated the two graphs above. The top one--whites--shows that 2-3 times a week is modal for couples in their twenties, but "weekly" becomes modal by the time the couple is in their forties. and even when it's modal, it's still well under 50%. And even among twenty-somethings, there's a detectable number who have sex once a month or less.
You can see even more of those types among blacks (lower graph). But, on the other hand, 2-3 times a week is modal until married blacks are retirement age.
I also looked to see if men were giving different stories than women. For whites, the only difference was that elderly men are less willing to admit they never have sex. For blacks, more men of all ages report that they have sex four or more times a week and fewer admit to having sex once a month or less. This could be evidence for male cheating--the question asks about frequency of sex, but does not specify with one's spouse--or just exaggeration.