Actually "A gun in your pants" is the alternative to "Happy to see her". A natural male physical reaction to the presence of a girl he like the looks of.
I will be so happy to explain whatever it is you wish. But you just explained it to ME. So why would you need anything explained after you explained it? I am just remarking that I was confused as to why you would explain it to me when I was the one who made the joke about it in the first place?
Does that make sense? Or am I misunderstand you? Maybe I have it wrong here.