Stories like this are what remind me why Snape/Black is my OTP. :)
The level of thought and detail that went into this story are impressive and I love that the sex -isn't- the main focus but rather it's just a really well told story that just happens to have wonderful sex scenes in it.
The development of the relationship is gradual and believable and Snape's reservations about letting Sirius close to his heart are just so true to the character. You really nailed what makes Snape so appealing and frustrating at the same time. I also love the development of Sirius as a more mature but still cheeky and slightly reckless man. He's still Sirius but changed in a way that is credible for how someone would be after a close brush with (permanent) death.
It would be very easy for me to gush (I probably already have) since there is a lot here that just made me smile but I'll just say this was a really nice way to end what has been an otherwise trying day. Thank you for this!