This image is Bill and Emily from Michael Rosen's book Vanilla Sex.  You can get a free download of the entire PDF book at www.michaelrosen.com. Select "Vanilla Sex" under GALLERIES in the left column, then click on "downloadable PDF" on the Vanilla Sex page, and then click "Download" on the download page if you agree to the licensing terms. - Image by Michael Rosen
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Vasocongestion

An accumulation of blood in the genital region (causing swelling or erection) due to sexual excitation.