[Steve's been more attracted to Loki than he's realized. A lot more attracted. He wasn't exactly delighted about him when the Avengers were fighting him, sure... but he's Thor's brother. And Thor trusts him, even if he doesn't trust him as far as he could throw him (and Thor can throw Loki a long way).

More importantly, Thor's talked about him, here and there. On those late nights when it's just been four, or three, or two of them. Sitting up late, watching the stars, getting into the deeper matters. Thor's talked about Loki. Doesn't mean that Thor sees everything there is to see about Loki. No one ever sees absolutely everything about another person. But it's given Steve a much better idea about the shape of the man underneath everything that's been done to him, everything that's happened to him. Talking to Loki himself has cemented that.

Seeing the humor in him. The care. The shyness. The strength to continue when others might just give up. Steve gets it.

So he didn't particularly mean to kiss Loki just then, but he doesn't regret it. When Loki kisses him back he makes a soft gasp of want, leaning into him. Loki's fingers feel good.]