If we limit our discussion to the U.S., then your "guesstimate" is overly optimistic. The actual statistics prove you wrong.
Table 3. Frequency of oral, vaginal, and anal sex in the past 12 months and past 30 days.
And it should be noted that these figures pertain to all men and women aged 18 to 70, not just married persons.
Regards,
Talking about the world.
Its common enough.