As is evident from the way it becomes densensitized after circumcision, the Glans of the penis is pretty resilient.
However, the internal skin of the foreskin is as delicate as any other skin which is kept moist. If saliva or sperm is left around the head of the penis or trapped under the foreskin after either masturbation or intercourse, the enzymes contained in those substances can cause a reaction in the elastic structure of that delicate skin making it tighten and encouraging tearing if it is stretched back over the Glans without care.
Frequent love making, dry masturbation or just a nocturnal erection can all cause fissures in the internal surface of the foreskin. Normally these present as parallel purple lines which radiate out from the head of the penis to the external rim of the inside of the foreskin and can take the form of sub dermal splits or actual surface tears.
The latter can be extremely painful and should be cleaned regularly using just warm water but, if inflammation or infection occur, you should see your healthcare professional straightaway. The sub-dermal cracks usually only make themselves known if the foreskin is retracted when dry.
Sometimes matters can be improved if you use a condom when making love. This helps by putting a protective barrier between the foreskin and the blunt force trauma which is involved in intercourse but also by stopping the resulting bodily fluids from collecting beneath it.
Candida is a frequent contributor to the cause of such cuts. This is not always down to sexual relations with a partner who has thrush. It is a type of yeast infection that can be systemic throughout the body due to a fungal overgrowth in the sufferer’s own digestive system. This can stem from prolonged use of antibiotics or oral steroids because these medications interfere with the body’s natural ecosystem and cause an imbalance in the immune system, allowing the naturally occurring Candida to run riot.
If problems with foreskin fissures recur, it might be wise to investigate a diet that precludes the foodstuffs which encourage Candida overgrowth.
Further to Kevin’s comment, I did some research on links between previously undiagnosed diabetes and men with foreskin fissures.
I found this research studying the links between glycaemic disorders and foreskin cracks.
If you are suffering with frequent fissures, it would be wise to visit your GP for some tests to check your blood sugar levels.