Can say, for a first: Bit long. But, manages to keep it together relatively well.
The style honestly seems to be a bit all over the place.
Like, Marceline seems to be draw almost by an entirely different artist. Same with Finn. It's just very... all over the place.
Likewise with how Marcy has a flat, hentai vag. But then Finn has a hyper dick.
And around the 2 minute mark,it seems like Marcy's pussy is upside down? But I'm not 100.
The music doesn't really add much tbh. It likewise feels a little out of place. Bit loud as well.
And then it kinda just... ends? Finn cums, leaves Marcy hanging, she looks pissed. Then stands about, then oddly waves at him?
Apart from knowing that I just watched porn, I honestly don't know what this was supposed to be, outside of porn. It's very disjointed and rather "just there". Despite seeming to try and tell a story.
Anyhow, I think it has potential. But, I think you need to focus what you aim to be doing. As, currently at least, it is very all over the place.