Disclaimer: Premise and characters borrowed from "Buffy the Vampire Slayer" and "Angel".
I understand why the Soldier-boy does it, he think Buffy wants that touch of darkness that he lacks. Nothing he can do will give it to him, but he's willing to try osmosis.
Captain Cardboard believes it was Angel's darkness that made his connection with her so strong. He can't see it was actually Angel's loneliness. She's the Slayer, he's the vampire with a soul, both of them utterly unique and ultimately alone. She has her little friends and now so does he, but those friends can't quite comprehend having the safety of the world resting on her slight shoulders ever night until she dies and his don't understand being neither human nor vampire. But he understood her, he understood being profoundly alone. Thanks to the chip I understand now as well.
Agent All-American, Boy-Next-Door, Riley Finn will never understand that. He's never been the outcast, not even in the piddly little ways Buffy's chums weren't of the in crowd in High School. Soldier-boy got one little taste of being on the outside looking in and he ran. Ran to his two-bit vampire whore then ran back to his military buddies.
All because she didn't love him enough to make him feel like he belonged, because the commando couldn't touch her in the way that the souled vampire had, because his perfectly normal life hadn't given him the keys to understand her heart, and he blames her for that.
So he went to his hookers, who needed him, and that made him feel complete. Sure they needed him, like he needs a hamburger. Any one else who thinks the boy is a bloody moron, just raise your hand.
Soldier-boy loves what he sees of her with all his heart, but because she can't do the same he cheated on her. If he thinks his situation was unique he has blinders on. I loved Drucillia with everything I was, but she always preferred another. More of less one soul, the same other that Buffy wants.
A hundred and twenty-five years I was second in Dru's heart while she consumed mine and I was faithful to her. Even when she was in Angelus' bed more nights than not, I never went to another. Yet the hypocritical little bitch throws me out for even thinking of another woman. My beloved dark princess fucked Angelus every chance she got and I never said a word, but she ended our relationship because I dared to admire an opponent I couldn't beat.
I took up with Harm to clear my head of the Slayer, to fill the emptiness Dru left in my life. She's a poor substitute for either. A little blond thing who's mouth is full of the Slayer's way of talking, but she's a hollow shell. No depth to my little Harm, and to my horror, it's the insides I want, not just the Slayer's utterly delectable body. Harm's presence in my bed does nothing to fill the emptiness in my life. I want another relationship, not a fuck-toy.
So I took Buffy to see her boy's new amusement. I guess I expected the hurt to make her fall into my arms. If she'd let me in, I could fill the empty places the Commando never even sees, and she could fill my emptiness. I can see into her heart, I understand being alone.
I could match her as Angel did, only more so. Angel doesn't need another to complete him, he's learned to survive loneliness. I haven't and neither has she. We both need a second to be whole.
Buffy is afraid to be alone, but all she had to do is open her eyes to the possibility and I'd stay with her forever.