"Flying the Flag at Half-Mast" also could refer to a semi-erect state of the male genitalia, the same one referred to as "Pitching a Tent".

Having it "hoisted to the peak for an instant and then lowered to the half-staff position" works like this:

Terribly hot woman walks by, and you start considering it. Then you notice the child, but, man, that lady still is pretty attractive... and the only thing left to do is make sure that it is "again raised to the peak before it is lowered for the day", usually in the comfort of your own bedroom.

Ouroboros adds: "I had thought that Flying the Flag at Half-Mast was a euphemism for not being able to get it up"

which is also an acceptable use of the term. It could also refer to one in the process of "limp-fucking", such as Ron Jeremy is notorious for.