Dark skin on the inner thighs is an embarrassing issue for both men and also women.
This problem even affects people with fair skin. It is actually more evident in people with fair skin. Imagine a young man or woman that always dresses fly but has very dark inner thighs, the day people see it is the day he or she loses her respect. Imagine you cannot go to the beach to show off your body. Obese people are more prone to having dark inner thighs than others because their thighs tend to rub against each other as they walk or exercise.
Some common causes are;
Friction between thighs when walking or exercising
Excessive sweating between the thighs
Exposure to harmful ultraviolet (UV) sunrays
Use of chemical-enriched products
Side effects of certain medications
Shaving or using other bikini hair removal techniques
An hormonal imbalance, polycystic ovarian syndrome (PCOS) or diabetes
In addition, the inner thighs are one of the most ignored body parts and people do not give much importance to their care. If you wish to brighten up your thighs to look good in your shorts and skirts, you can do it easily at home. There are so many natural and safe ways to lighten the skin.

http://www.confamgist.com.ng/2016/01/ways-to-lighten-your-dark-inner-thighs.html

In the summer, I always get heat rash on my inner thighs and it leaves a dark area after it's healed that stays for a while. Dermalmd Intimate Lightening Serum is the only product I've used that helps lighten this area. Love this serum!