How often do couples (including married couples) have unprotected sex?

Not if they don't want kids. Most married couples just take the pill rather than condoms though.

It is common for the girl to be on birth control, at least in my situation, and it is always the pill. Condoms are a nuisance, and in college, I used them with a particular girl, and as far as we were concerned, it was a waste of effort, because big loads tended to lead to slip offs.

It depends on what you mean by unprotected sex.
If you mean without a condom most couples just use birth control.
If you mean without any form of contraceptive than usually only teenagers or couples trying to have kids do that.

If ANY couple is having unprotected sex, then by definition they are trying for a baby. And given that the average couple in most western societies has one or two children, that would mean that unprotected sex is quite infrequent.

Couples (including married couples) do not often have unprotected sex. If they did, the family size would be roughly six or nine children per couple.

In our society today younger women tend to finish high school and earn an income; both of those are harder to do if you have a child. Oddly enough a woman is much more fertile at the age of 15 than she is at the age of 35. The structure of our economy and educational system play a very large role in term of people having unprotected sex.

Probably not all that much. I know the wife and I always used some sort of birth control except when we were trying for a kid.

I never had unprotected sex until I was TTC. I was 31 when I first attempted to get pregnant, started having sex a few months short of 17, met my husband at 21, not once until TTC was it unprotected.