Take it up with Virginia; age of consent there is 15. Intercourse with a minor above 15 as an adult can still give you statutory rape charges, as referenced here: "The age of consent for intercourse is 15 years old in Virginia; however, there are other sex acts that set the age at 18. These acts include cunnilingus, fellatio, and anal intercourse, which means that minors cannot legally consent to certain sexual activity. Additionally, if you are an adult and have sex with a person under 18 years of age, you may be prosecuted for statutory rape. Statutory rape is generally charged as a Class 4 Felony, but the exact charge and penalties will depend on all the factors and ages involved."
It isn't always the case, generally age of consent precludes it, but not always. Age of consent isn't a get out of jail free card. Additionally, it doesn't specify about relations between people in positions of power.
It's all moot anyway, Roiland lives in CA, and he was inviting a 17 year old to his house in Los Angeles. Age of consent there is 18. Did he assault her? No, but what 36 year old man is inviting a 17 year old to his house? Multiple other reports of girls at 18, still in high school, also receiving advances from him.