I say both men and women are understating their number of sexual partners
Contrary to what is portrayed in this chart, I postulate an American
male average of about four.  I do not agree with the common claim that
American men will overstate their number of partners.

In the GSS, males report an average of 14.19, women an average of 4.76.*  If you mean the median, then males report a median of 3, woman a median of 2.  For men, 4 partners is the 62nd percentile.  So perhaps the results are compatible with your guesses.

P.S. The modality of monogamy is not an artifact of age.  Monogamy is modal for 25-44, 30+, and 40+, for both men and women.

Update: Fans have calculated the total number of partners for the main characters on Friends.  It’s high.

* Excluding the “989 or higher” bin.