Marco is basically the secondary protagonist of the show. He's the normal everydayish dude to star's manic pixie dream girl. Everyone wants his dick for weird reasons (though only 3 of them are DTF for sure, the others are just kinda hinted at.)
The girls are as follows:
Jackie (skater girl) is Marco's girlfriend and the one he'd been crushing on since kindergarden.
Star realized at the end of the last season that she's in love with Marco
Hekkapoo liked the man he grew up into after trapping him in a pocket dimension for decades (and gave him a way to bootycall her)
Janna (girl with the beanie) has a weird fixation on marco and his stuff, but hasn't made any overt advances.
Tom (star's devil ex) is often paired with him because Yaoi but they hate each other... slightly less than they used to but still there's a burning hatred.