An old myth goes that one hundred men were interviewed about whether they masturbated, or not. It is said that eighty five per cent admitted that they did, and the other fifteen were liars. Listen, it's yours, so you do what you want with it, but take care of it because it's delicate and it doesn't take much rough usage to cause cracks or cuts on the foreskin. Men with a foreskin need to pay more attention to their personal hygiene. It should be retracted and the whole area cleaned at least once every day, for health reasons as well as standard personal grooming. Whether you have a sexual partner, masturbate, or neither, you should pay attention to the inside surface of the foreskin. It is possible that even men who do not pursue personal or partnered sexual relief will still experience erection during sleep, so all men with an intact foreskin should examine it.
Making love frequently with a partner, re-establishing sexual contact after a period of abstinence, dry masturbation, using saliva as a lubricant, or simple nocturnal erections can all cause fissures in the internal surface of the foreskin. They usually take the form of purple parallel lines radiating outwards from the head of the penis to the external rim of the inside of the foreskin. They can be actual surface cracks, or sub-dermal splits. You'll know if they are actual splits as they sting quite severely. The sub-dermal fissures only hurt when the foreskin is retracted when dry. If they are surface cracks, clean it often - but without soap or other wash products as they only make it worse. Just use warm water, and if swelling or infection set in, see a doctor immediately.
They can be caused by fungal growth from thrush, even if you are not in contact with a sexual partner. Heavy or prolonged courses of antibiotics, or excessive use of oral steroids for conditions such as asthma can create oral thrush quite easily, and using saliva as a masturbatory lubricant will often transfer the condition to the penis. You will usually have a persistent itch around the foreskin as a first sign, and the inside of the foreskin where it's attached to the head of the penis (the Glans) will be bright pink or red even if there are no fissures present. You can confirm the presence of thrush in your system by looking at the inside of your throat in a mirror. If you have white patches on either side of the throat walls, strange patterns in the skin of the surface of the tongue, or what looks like small pieces of chewed white nuts stuck in the folds of the throat lining, you need to see a doctor for an antifungal treatment.
However, you should also address the possibility that this manifestation in both mouth and penis could stem from a systemic Candida infection that started elsewhere in the body and investigate an anti-Candida diet if the problem recurs.
Just using saliva as a lubricant in masturbation, or not washing the whole area with the foreskin drawn back after ejaculation whether saliva has been used or not can also cause irritation, even without a fungal infection. This is due to enzyme action. Enzymes are contained within saliva as a part of your digestive process. They are also present in sperm. The sperm is like a tadpole torpedo carrying a payload of DNA to the egg. On contact, the sperm breaks open its warhead, and releases enzymes to eat a hole in the surface of the egg to allow it to deliver its payload. If saliva or sperm is left around the head of the penis after either masturbation or partner sex, and trapped underneath a foreskin, the enzymes will tend to begin to act on the surface of the Glans, and the internal surface of the foreskin.
The Glans is pretty resilient, as is evident from the way it becomes desensitized after circumcision, but the internal skin of the foreskin is as delicate as any other skin kept moist. The enzyme action can cause it to thin and lose some of its elastic structure. This in turn will cause it to tighten, and can encourage tearing when stretching it back over the Glans without care.
Fissures can appear even without sexual activity. Most men experience erections in their sleep at some point during the night, even if unaware of them. The foreskin and Glans having been rubbed on the sheets with movement during sleep will be absolutely dry. This can cause the foreskin to stretch violently, and the sub-dermal fissures can easily result.
Whether sub-dermal, or open on the surface, a good hygiene routine will ensure that at worst the condition is a mild and temporary affliction. If there is any sign of oozing, severe redness and inflammation or severe pain, don't mess around, consult a doctor immediately. There's no need for embarrassment, yours is unlikely to be the first they've ever seen.
As I said at the beginning, it's yours, you only get one, look after it.