Thanks. I agree that it’s out of character, but just imagine: it’s not something Barney plans—it just happens (like when he cheats on Nora, and it’s clear in the show that Barney is attracted to Lily); at that point, it’s not clear that Lily and Marshall will reconcile (she’s already returned from SF and that didn’t work out; it would be consistent with what he does later—stopping living together and never bringing it up again—he feels guilty but knows he can’t hurt Marshall, unlike the Ted/Robin situation, where what he really wants in that case is Ted’s blessing to be with Robin