As Jon mentioned, this reads very much as male-voiced, even though your PoV-narrator is supposedly Natalie.
There are also a lot of the common mistakes that get flagged in most feedback requests. So, rather than list a ton of them, I'll just break down your opening lines…
'Natalie, is this a picture of your bum?’
Too verbose. "A picture of" is irrelevant. Who would say that? It is implied. If you were holding up a picture of something, asking about it, you wouldn't reference the fact that it is a picture of the subject. You would ask if it is the subject. Direct. Clean.
Also, it's flat. There's no personality in this. How is he asking the question? Is he leering? Is he excited? Ashamed to be asking? Nervous? Most of which would simplify to "Natalie, is this…?" plus an action.
Hard to argue with a single-word line like this, though depending whether the subject is named in the previous line, a stronger reaction may be required. If not, a reaction to the body language in Steve's behaviour.
My boss Steve slid an A4 piece of paper across his desk toward me.
Argh. You said this isn't the beginning of the book. So we can assume Steve has been introduced elsewhere. There's no need to tell us he's her boss. There are other spots where this could be inserted, such as when she's considering him.
Likewise, telling us the paper is A4 is a waste. It's a photocopy (ok, we only learn that in the next paragraph, but it's quickly evident) so the size is assumed. And no one will have a piece of paper shoved towards them and think: oh, wow, that's A4-sized. It's just a piece of paper.
‘Does this derriere belong to you?’
Belongs on the same line as his action.
I peered at the grainy photocopy not quite knowing how to respond. ….
Peer? That's a studious action. It's utterly incongruous here. (It's an example of the male-voice coming through. A man might, even when not wanting to display such, peer at a photocopy of a woman's rear end.)
And grainy? Maybe if this was set in the '80s when photocopies were grainy. But that's not a reality of modern offices.
We are also lacking the kind of visceral reaction that a woman will feel if her boss calls her into the office and basically asks: "Are you a slut?"
Yes, I've just ripped apart the five opening sentences. And it all, pretty much, comes down to believability. Or the lack thereof. You need to really get inside your PoV's head to make a scene like this work. You also need to get inside Steve's head. And then step outside both to analise how those two forces would impact each other when a photocopy of an arse shows up.
And while your twist at the end is interesting, a good zinger, it likewise fails the believability test. That is a detail that would very much play into Natalie's reactions and responses to Steve. She would be thinking about that from the very beginning. As soon as Natalie knows what the initial question is about, her daliance with Steve will be a key driver of the conversation.