I'm understanding that sexual activity between a person over 18 and a person under 16 is considered statutory rape in the state of Washington. What I haven't found the answer to is if kissing is considered sexual activity? Is there a certain point it becomes illegal or does the state consider any lip connection sexual activity?

Sexual Offense definitions under Washington state criminal code could be considered "broad", especially the term "sexual contact". Ultimately, this type of ruling would have to be based on a judges interpretation and would likely be determined on a case by case basis.

RCW 9A.44.010 Definitions.

(1) "Sexual intercourse"
(a) has its ordinary meaning and occurs upon any penetration, however slight, and

(b) Also means any penetration of the vagina or anus however slight, by an object, when committed on one person by another, whether such persons are of the same or opposite sex, except when such penetration is accomplished for medically recognized treatment or diagnostic purposes, and

(c) Also means any act of sexual contact between persons involving the sex organs of one person and the mouth or anus of another whether such persons are of the same or opposite sex.

(2) "Sexual contact" means any touching of the sexual or other intimate parts of a person done for the purpose of gratifying sexual desire of either party or a third party.