Jim agrees with Michael about Pam's boobs (but not that they're the best things about her, because he's really not that sleezy, current circumstances notwithstanding) because they are really, really nice.

And maybe he should be thinking about his girlfriend right now, while he's got one hand wrapped tightly around himself, stroking upward (faster then slower, squeezing the tip and his hips jerk) but she's not here, and her boobs aren't... like Pam's.

He's a guy.

He's seen his share of porn, and he knows that girls who look like Karen don't (can't) wrap their breasts around a guy's dick.

(He closes his eyes and thinks about how that would feel if it were her and he's coming all over his hand almost before the thought is formed.)

Jim is a really nice guy.

But yeah, he's a guy.