I think it's a combination of things. He tells us he was bored that night and when he saw Justin, he KNEW he was a virgin...something different for him perhaps. Plus, I think he was impressed with his bravado. Then, Gus is born and Justin and Brian bond a little. I don't think there was ever a conscious decision on Brian's part about anything that happened that night, beyond him bringing Justin home with him.
I think Justin, like Gus that night, got in under the wire (like Deb says). How could he not? LOL
As for your ETA: We don't hear about a one-fuck-only policy until 201. He fucks people more than once before that. Look at Kip Thomas...and he tells us that or it's inferred a couple of times. But, the actual policy, doesn't come into play until after the bashing.