In Ontario, Canada, where I live, it is not illegal for an 18 year old to be sexually active with someone 3 years younger. In most states, I believe, similar laws also allow this. But it really does depend on jurisdiction. In some states, a person is not even legally an adult until they are 21. Here, the legal age of consent is 16, except where the older person involved in a relationship with the younger person is an authority figure such as a teacher or coach.
The letter of the law may be being followed in this instance, but certainly not the spirit of the law. A judge looks at both when deciding the case, and if the judge looks at the merits of the case, I'm almost certain the charges against the girl will be reduced or dismissed all-together.
The kiss itself is not a sexual act unless the manner in which the contact is made is for sexual purposes. So in that way, I agree. A kiss on the lips, cheek, or forehead does not, in of itself, mean the two people are engaged in a sexual act.
Two people making out on a bench in a part are clearly engaged in something other than an affectionate kiss. Likewise, an adult feeling up a child and kissing the child is clearly doing so for sexual purposes. But we have to be careful that we don't automatically jump to a conclusion when we see something like this. Too many people have had their lives ruined over allegations of sexual improprieties when the act in which they were engaged in was completely innocent.
It's gotten to such a degree that the age-old custom of parents taking pictures of their children taking a bath can be charged with creating child porn. Or when a 16 year old girl takes a picture of her topless and sends it to her boyfriend is charged with distributing child porn. It's all ridiculous.