There are sexually transmitted diseases that are not always infections and that do not always have any serious consequence, but they should be treated for the sake of a comfortable sex life.
The most widely known such disease is Candida overgrowth. It is deliberately not called infection, since it is only an imbalance in the body. Its reasons can be stress, weak immune system, diet full of man-made, unnatural and artificial additives or sugar just to mention a few. However, it can spread from woman to man via sexual intercourse, and it will feel like a fungal infection on the penis, with itchy red spots all over it. For the woman, it means a thick discharge, dry vaginal wall and an itchy sensation on the minor lips and inside the vagina. Obviously, sex is nor comfortable with these conditions, and if lubricants are used to overcome the symptoms, they might even make it worse with allergic reactions on the mucous membrane.
Another imbalance can occur in the vagina, especially due to not properly cleaned toys, or inappropriate sexual practices with anal sex, that is, the penis is inserted back into the vagina after it has been pulled out of the anus, without cleaning it, so bacteria from the colon are transmitted to the vagina. And this will lead to an infection that can spread from woman to man, as well.
HSV or herpes simplex infections are also not deadly, but they are still responsible for uncomfortable sex. Unfortunately, they cannot be eliminated from the body, so the best is to protect our partner if we have it by not having sex during an outbreak. Keep in mind that condoms do not protect against genital herpes in most cases.