at least when it's "a lengthy, 'good,' 'deep,' 'passionate,' 'intimate,' 'romantic,' and 'memorable' french kiss in the bed of the defendant after an overnight stay, and the kiss achieved emotional arousal and was followed by professions of true love and repeated encounters involving the same conduct." So says the Kansas Court of Appeals, though without word on what you're allowed to do if there's no overnight stay, or if it was followed by professions of something other than true love.
This case involved a female high school teacher and her 16-year-old student; in Kansas, the age of consent is generally 16, but there's a special rule for teacher-student relationships when the student is 16 or 17. However, the same would apply to two 15-year-olds French kissing, which is also a felony; it's a lower-grade felony than full-on statutory rape, but it's a crime nonetheless. Presumably in states where the age of consent is 18, two 17-year-olds could be similarly punished, at least if the statute uses similar terms ("lewd fondling or touching").