What is it about Marco? Even when he's not crossdressing, even when he's acting manly, even in the Heckapoo episode, there's just something feminine about him. Some of it is how he acts--like the way he wraps a towel about him--but it's more than that. There's just something about his design and his character that feels womanly to me, even though he's clearly straight.
And clearly the showrunners agree, given how much fun they've had with the idea.