The final episode for S2 was giving us an interesting point of view, Marco had grown a lot confident with Jackie, he wasn't feeling things for star as some want to believe, he was just worried about how to react to it. Fuck they say it in the episode! He doesn't want things to change.
Good thing is Jackie pulled him out of those thoughts a couple of times without problems, the body language alone and the way they talked... I don't know how much of a virgin you are but I'm sure many have been in that situation.
End of the year party, teenage life, yeah, Jackie and Marco were already doing it, for christ sake they are teenagers there is no other thing they would do.>The way he is hugging her>The way he is daydreamming>The way she wants him to read her mind
those are signs of a girl and a guy that have already make a physicial connection, intercourse, or if you want it clearer, they had already fucked.
Now, some may argue "that's your wet dream, self insert". I will say, no, my younger days were better cause I didn't have a moronic friend, but I was a teenager as well as Daron, I went to parties and were somewhat bullied, we all were on a certain degree, so I and you adultanons, know how things are, an see those details and know them to be true. We all know that we have been in a similar situation and between an autist and a hot girl we had a crush for the hot girl, some did manage to go out with her, some didn't. Those who had her as GF didn't change her for someone worse.