Bonobos, also called pygmy chimps, are close genetic relatives of chimpanzees and humans. In fact, bonobos are as closely related to humans in terms of DNA as they are to chimpanzees. Although they look a lot like chimps to the untrained eye, there are some marked differences between the them. They've got more human-like faces and expressions, including bright red lips. Bonobos are also gentler with each other. Unlike chimpanzees, bonobos have never been known to engage in gang warfare or organized hunts. They live relaxed lives by comparison even though they also live in the same large social groups as chimpanzees, along with all of the stressors - food competition, social hierarchy, and the like.
Rather than allowing competition to boil over into aggression, bonobos "make love, not war" as one well-known primatologist (Frans de Waal) has said of them. Basically bonobos are a very physically affectionate bunch. They kiss each other open-mouthed, including with tongue. They have sex, including oral, with each other without regard to age or who is male and who is female. In other words, homosexuality and pedophilia are the norm.
Some scientists have speculated that bonobos are "paedomorphic" which is to say that they retain infantile or youthful traits into adulthood. Paedomorphic traits are those that are characteristic of infants and children. Humans are also believed by some to be paedomorphic. For a comprehensive understanding of the issue, Stephen Jay Gould's 1977 book Ontogeny and Phylogeny is a must read.
I believe there is no underlying evolutionary difference between open-mouth exploration of other's bodies, including "French kissing" and erotic kissing, and that of the non-erotic "affectionate" kissing done with closed lips. I think we begin life prepared to do the first and only learn later that our culture reserves that form of affectionate expression and sensation-seeking for those we are romantically involved with. The rules of display vary immensely from culture to culture and even in sub-cultures. Some allow open-mouthed kissing in public; others expressly forbid even closed-mouth kissing in public. Some allow open-mouthed kissing between people of the same sex; others forbid it and punish it with death. Most forbid open-mouthed kissing between an adult and a child. Yet, it still occurs.