Another possible interpretation of events: Lestrade got a message from someone at the military base (Mycroft's assistant) that Sherlock Holmes had been apprehended and needed to be taken away from the area for his own safety. Lestrade immediately thought "oh crap". He had a few more days leave, so he took them, citing marriage issues. As soon as Sherlock mentioned Mycroft, though, he realised he'd been played. Hence the bitterness and the denial.
Obviously my headcanon is somewhere in between this and fanon's determination that they discussed it in between having sex.
Re: What show were you watching?
Another possible interpretation of events:
Lestrade got a message from someone at the military base (Mycroft's assistant) that Sherlock Holmes had been apprehended and needed to be taken away from the area for his own safety. Lestrade immediately thought "oh crap". He had a few more days leave, so he took them, citing marriage issues. As soon as Sherlock mentioned Mycroft, though, he realised he'd been played. Hence the bitterness and the denial.
Obviously my headcanon is somewhere in between this and fanon's determination that they discussed it in between having sex.