Marlene McKinnon and Sirius are dating to the public eye of the students in Hogwarts and the professors. But in reality, it's a cover. For both of them. Marlene being a lesbian, and Sirius being gay....and dating you.

A Slytherin. Friends with Regulus.

It's a secret, because Sirius' family hate him already, due to him getting sorted into Gryffindor. Imagine their reaction to him taking it up the ass.

You need the cover too. You're parents are Slytherins and despise the thought of Sirius.

You and Sirius were hooking up in the Gryffindor common room, when all of a sudden Dumbledore tells everyone to meet him in the great hall.

You and Sirius fumble for your clothes and run to the great hall

You get there and sit with the other Slytherins, you sit next to Regulus, panting from running.

He grimaces, looking down at your tie.

You look down and see...

Gryffindor.

You look at the Gryffindor table and see James snickering at Sirius tie. You look at it. Sure enough.

Slytherin.