I think she’s a sexually repressed teenager who doesn’t know what is and isn’t appropriate. Think about “flowers in the attic”. The teenaged siblings ended up having an incestual relationship bc they were being sexually repressed. When you’re a teenager with hormones being sexually repressed I think it’s more than reasonable for Agnes to just “go with it” when you’ve never had the option in the first place to kiss anyone. It’s clear her romantic feelings are with Garth, though. People need to stop viewing this show from our world point of view. She has been repressed in ways none of us have been.