Washington State law defines sexual intercourse as "ANY penetration, however slight."

RCW 9A.44.040
Rape in the first degree.

(1) A person is guilty of rape in the first degree when such person engages in sexual intercourse with another person by forcible compulsion where the perpetrator or an accessory:

(a) Uses or threatens to use a deadly weapon or what appears to be a deadly weapon; or

(b) Kidnaps the victim; or

(c) Inflicts serious physical injury, including but not limited to physical injury which renders the victim unconscious; or

(d) Feloniously enters into the building or vehicle where the victim is situated.

(2) Rape in the first degree is a class A felony.

RCW 9A.44.010
Definitions.

As used in this chapter:

(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.

http://apps.leg.wa.gov/RCW/default.aspx?cite=9A.44.040