Even back when he was still trying to salvage his marriage, Lestrade use to get off to fantasies about Molly and Sherlock together. He could admit to himself that he had a bit of an attraction to Molly, but Sherlock? That was a bit much. Still, he could justify the fantasies as being "safe", since he was married and all and might've felt guilty fantasizing about fucking her himself.

The truth is, though, that he wants them both. And now that Molly's no longer engaged and Sherlock is alive again, he might have a chance, if only someone could give him the right push.

(Bonus points: Donovan, Anderson, and/or John push him to it, but confuse the matter, either because two of them are pushing him towards different people, or because one of them doesn't know which of them he wants.)