OP here. I can't believe this doesn't have more comments. I love the premise that Sirius doesn't penetrate women because he doesn't to pass on the family line.

The absolute trust and sensuality that Remus and Sirius are sharing here is stunning. Remus is doing all the things Sirius wants to but won't. There is a give and take between them that is at once just fucking beautiful and also hot as hell.

This bit here:
He tongued one nipple lightly, afraid to bite, even gently. Just as Sirius feared children, Remus feared another form of offspring. They understood each other in this. But Dora didn't. Her hands flew to Remus's hair. "Harder," she hissed.

"I can't," Remus answered. There was a note of disappointment in his voice, and she opened her eyes once again to meet his.

"Right. Sorry. I forgot." Thrilling, really, to Remus, that anyone would ever forget.

This admission of hers and his reaction is at once heart warming and heart breaking. Poor Remus.

And this too:

Both, though, understood now that they would not be alone together forever. Someone else had stepped irrevocably into their relationship.

I love Tonks, and this is such a nice way to show how it might have happened. I'm so glad you chose her.

Wonderful anon. I really, really loved it!