I think the reason for the ”dudebro” side of Dan in relation to Amy is because she’s pretty much the only woman who gets to know him exactly as he is. He’s not trying to charm her or impress her. Since the first ep (so since they had their little romance), he’s been honest with her about what he gets up to with women and his various other slimey pursuits. And with more serious stuff, eg his breakdown, not being good with feelings. The only thing he’s reallytried to hide from her is his jealousy
I don’t really buy that, I’m afraid – or at least, not entirely – it’s too strategic for it to just be about his comfort with her.
Notice how he doesn’t look at her once during that little speech? He tends to pull that douche, overshare thing (think how he was about Sophie in 5.09) when he specifically wants to shut down Amy’s potentially upset reaction. Like, crass comments have always been one of his things, but they’re usually asides – he didn’t aim this kind of thing at her earlier on in the show.
Now, I don’t think he quite realises that Amy perceives it as a more…emphatic shutdown than he intends. She reads his rambling about how “hot” other women are as a not so subtle message that she isn’t – which I don’t think is actually the message he means to convey.
But I think he knows that it’s an effective way of shutting Amy up and preventing her from fighting back – because the mere fact of him taking to her like that makes her seem unreasonable for imagining there’s anything meaningful between them. And in a moment where he wants to tell her (relatively kindly by his standards, I think, which says nothing good about him) that he’s not going to support her and expects her to get an abortion, he really, really wants to stop her from drawing the conversation out. Why do you think he climbed into the elevator so quickly?
And I think he’s hidden a lot more than jealousy from her – in particular he’s managed to hide the fact that he cares about her, and is attracted to her, so well that even after knowing each other (and being friendly for most of) six or seven years, Amy’s still terrrified he’ll say something cruel to her when she tells him she wants to have his baby. His performance of indifference has fooled her, if no one else.