This may be a mild irritation caused by vigorous sexual activity or a reaction to certain substances found in saliva and body fluid. It may also be due to heat, a fungal infection and dermatitic skin condition. This may not be something serious. Wash the area after urination or sexual activity. Wash using a mild cleanser .Be careful not to irritate the area. Protected sexual activity should be observed.
Watch out for any pain or burning in urination as well as any form of redness, blistering or discharge in the area.
Some other clues. I have recently been away for 1 month with no sex, no irritation. Upon arriving home, after oral sex only, irritation returns. We have both been tested vigorously over the last 6 months for sti's. She has hsv2 genital. I don't. There has only been saliva contact since returning. The skin does not get dry, and redness does not occur, but sensitivity and irritation happens, and I get sensitive to fabric rubbing with this condition.