Because she tells it best

Statuory rape is nonforcible sexual activity where one or both participants are under the age of consent. Wich in NY is 17. https://www.ageofconsent.net/states/new-york

On the roof Jenny did not agree so what Chuck did can be considered an assault/misdemenaor. And if she had agreed they both would’ve been guilty.

The ‘under age of consent’ part means that almost everything sexual our UPSers did during s1 is illegal. Shepard wedding, limo scene, Blairs misadventures with Nate and Chuck - all of it is punishable. And every cougar that slept with a horny Chuck before he was 17 is responsible for molestation in eyes of the law.

Back to Jenny. Now we just need to know if she turned seventeen by the end of s3. 

It is apparent that every ep has a strong correlation with the day it was aired. For example episodes of Blair’s BD fall in the same period of time in November (16,14, 17, 15, 16, 19).

According to that Jenny’s BD is sometime around March 30 (her 'Sweet Sixteen’ party in s2) and by the time of Dorota’s wedding she was legaly able to give her consent.

Which she did in May.

Also, according to GG Wiki Jenny was born on 3 April and Nate on 5, which means that by the time of his Assassin Party she was 17.

So Chuck-Jenny fiasco in s3 however unfortunate was comletely legal.

PS I don’t care if someone already said that.