When Spike realized Red went for birds (and not the bees), his first thought was "good on her." He was sick of watching her little self-involved scooby pals forget about the girl's heartbreak time and again. So now Red's got a dirty little secret that makes her happy and doesn't involve the others? Well, she was the smart one.
He'd always thought the girl deserved more, anyway. Maybe he didn't notice it at first, what with the super-strength slayer trying to kill him all the time, but by his third trip to Sunnydale it was clear which one of the lot had the compassion to be worth anything. Had Buffy baked everyone cookies after she made a mistake? Had she ever been upset - if just for a minute - that Spike couldn't bite her? (He wouldn't even get started on the English bookkeep or the...well, Xander...)
She was the only one worth anything, and she'd given up the men-folk for something new.
The men were going to be missing out.
When Spike realized Red went for birds (and not the bees), his second thought was really a bit different.