This is almost exactly how I found out my ex-wife was cheating. She bought a ticket to a concert and then freaked out when I bought one too. Why wouldn't I want to go to a concert with my wife? She said it was just going to be a thing with her friends from work. When we arrived there was exactly one friend from work. A guy friend.
thrust distance per coitus session: 13 cm x 300 thrusts x 2 (accounting for insertion & removal) = 78 m
coitus sessions to accumulate 5 km of thrusts: 5 km / 78 m = 64.1 -> 64 sessions of coitus
So OP’s wife would have to fuck 64 times to accumulate 5 km of cock. Even with amazing refractory periods, that’s still a minimum of 32 guys running a train on her.
Edit: working through this reminded me of that episode of HBO’s Silicon Valley where they started talking about handjobs and hotswapping dicks and hit on middle out compression.
Sir I think you are forgetting that you do not pull the pens completely out with every thrust unless you want to snap it off by accident. I think a 1.25 multiplier to your calculation is therefore necessary.