Roland Lamah opened the scoring for Swansea at the Liberty Stadium on Saturday in the second minute before Stephane Sessegnon and Youssouf Mulumbu found the net in an inspired second-half display by the visitors, who won for the first time in 10 games in all competitions.
And Monk bemoaned his players' attitude in the second 45 minutes having been totally dominant before the break.
"You never have it won at half-time, whatever game you play in," he said. "Maybe we should have scored a couple more, but we didn't and that's the way it went.
"The way we played first half is exactly how we wanted them to play, so we were disappointed not to take more from the half.
"If you've got them on the ropes and you don't knock them out, you're always open to them coming back into the match and that's exactly what happened.
"They got the second goal which gave them the impetus going forward.
"I don't think we deserved to lose the game, but we definitely didn't deserve to win it after that second-half performance.
"In halves - very good first half, very poor second half."
After seeing a West Brom side fighting for Premier League survival, Monk remarked that there were big lessons to be learned from their opponents' current plight, with Swansea sitting just four points clear of the relegation zone.
"The only positive for me is that our players have seen a team scrapping for their lives and they're going to have to match that week in week out now," he added.
"Before any football, before any tactics, before anything else, how much you want to win the game is the key.
"They've got it in them. I think it's a wake-up call for them, the second half especially."
