I suppose it was meant to show just how carried away individuals like him and Star can get, those who have a desire to stand out as an individual and all that. Given we don't know the specifics of what he and Star talked about, we can't come to any meaningful conclusions about what the two of them were expecting to happen.
Perhaps he assumed that, with Star's blessing, he'd be protected from rebuttal no matter the nature of the performance. Given how vaunted the princesses are in Mewnie culture this is the thinking that I most align with. The whole point of the song day is to promote what is to be the next ruler of the kingdom, as such by being in her good favour and doing what she asked (with some artistic license) he likely assumed he'd be protected from any direct repercussions. Had he not left things to the last minute then perhaps he may have realised how dangerous the whole idea was, particularly the last part which Star seemingly didn't discuss with him.
Therein lies his true mistake, it's likely he though he was doing her a favour. I mean in his eyes, why wouldn't Marco be interested in her? She's this seemingly amazing individual with all the wonders that Mewnie has to offer, surely he must just not know of Star's feelings and so he will do Star a favour and get the ball rolling. Howeve,r by not communicating this intention with Star he's dug his own grave as she's going to look to distance herself from him, throw him under the bus so to speak and say that what he revealed was not what she actually intended.
As such, he's got no real leg to stand on in defense, he'd likely be execute, yet he would live on in infamy if nothing else. It's an interesting little feature of the episode in my eyes.