Not to REALLY nitpick, but there's a cut not too long after he cums, afterwards she says "I make every guy cum early" (the cut was probably editing out the director suggesting she say it)... then they fuck some more with the load clearly still there... then there's ANOTHER cut, and after that the cum looks fake (and her tits are perfectly dry) and they continue fucking for the rest of the scene.
Rather than "being safe", I think it's more likely the guy lost his hardon at some point (which they edited out) and that's why they cut... the guy took a break for a while... and they knew there would be an obvious discontinuity after the break because the cum had dried, so they put some fake cum on her (you can see obvious fingerprint patterns) and tried (badly) to make it look seamless. What's kind of funny is she smears some of it on her right tit, as if she'd remembered it had some cum on it originally.
Still an extremely hot scene... I wish it had gone without interruption though.