There's lots of interesting discussion beneath the gifs, but the thing that stands out to me the most is the virgin/whore dichotomy. I don't have much to say here that hasn't already been said by others, but I wanted to hash it out for my own thoughts.
In Innocence, Angelus (I hate the retcon of soulless Angel going by a different name, but I will abide by it in order to be avoid confusion) rejects Buffy because she's a virgin and unskilled in bed. He mocks her naivety. In Dear Boy, Darla concludes that Angel is dismissing their relationship and denying that she ever made him happy because she is not "fresh". She bitterly generalizes it to all men: "Guy gets taste of something fresh and he thinks he's touching god."
The one identical theme in both scenes is Angel's mockery and dismissal of the woman's feelings. He tells Buffy, "Lighten up. It was a good time. It doesn't mean like we have to make a big deal." When Darla is angry and hurt by his claim that she never made him happy, he says, "You know - you are getting awfully bent over this, Darla."
Angel's criticisms of the two are a reverse. Buffy is rejected by Angelus because she is bad in bed, and her romanticism of their sexual experience is a source of contempt. Darla is good in bed, but is unable to give Angel true happiness. Darla clearly loves Angel - she even says as much back in BtVS 1.7 - but her love is not good enough for Angel (even though, in fact, she discovers in The Trial that he loves her deeply). This is ostensibly because she's a monster, but even that has questionable implications because of the Fallen Woman trope (see, for example, the Supernatural episode Blood Brother, and Benny the vampire's attitude towards his now-vampire formally-human girlfriend, Andrea). And given Angelus' obsession with destroying virgins and the fact that Darla was a literal whore, it's hard not to see his rejection of her as at least metaphorically due to her sexual history.
And oddly, for both Buffy and Darla, Angel's criticisms are hurtful because they want him to love them, to value them for more than just their sexuality. Angel/Angelus makes it sounds like it was just about the sex, that he was just using them. Yet Angelus' obsession with destroying Buffy and Angel's obsession with saving Darla reveal it was much more complicated. But their respective roles as the virgin in need of protection and the whore who tempts Angel/Angelus with dark excitement always, always play a role in their relationships with Angel and how he views them.
On a somewhat unrelated note, I've seen at least one person recently dismiss Buffy as being attracted to Riley because he "safe", implying that her heart of hearts lies with Angel and/or Spike and if she were true to herself she would take the risk of being with one of them.
Let me take a moment to note what "safe" means in this context, when comparing Riley to Angel or Spike. "Safe" means that Riley does not have a history of trying to kill Buffy and her friends. The he does not have the possibility of losing a soul at any moment and turning viciously against Buffy. That he does not have a history of stalking her, or trying to rape her. That he does not have a century plus of brutal and vicious murder in his past. I don't know about you, but I would call that "basic requirements for a relationship".
True desire and "safety" are not, actually, incompatible - unless you, like soulless Spike, are of the opinion that "Great love is wild ... and passionate and dangerous. It burns and consumes." Of course, finding the relationship you want may require you to step outside of your comfort zone (which in fact Buffy did with Riley, many times, especially in regards to his role in the initiative which she wasn't always thrilled about). But finding the relationship you want should not involve dating someone who is untrustworthy and dangerous. That may or may not lead to greater intensity - but it will definitely not lead to greater happiness. And as Buffy pointed out, that kind of love burns out. The intensity doesn't last.
And I would argue that the #1 reason Buffy/Riley broke up was because Riley convinced himself that true love had to have the kind of intensity and darkness that Buffy had with Angel. And so he went behind Buffy's back and try to be darker. Surprise, surprise, it backfired. Riley's betrayal led directly to the end of the relationship.