Despite what Tony thought, 'testing each other's weapons' was not a euphemism. As it happened, Natasha was quite fascinated with the sword Sif used, even more so when it extended to a spear. In turn Sif became quite fond of the small throwing knives the agent used, putting them to good use against their test dummy.
It is only when Natasha brings her guns into the equation that Sif appears to lose interest. Which, Natasha states, isn't fair because where Sif comes from they have magic, and firing a bullet at someone is the closest she will probably ever come to that.
"They are useful, I'm sure," Sif admits, "from a distance," and goes on to explain what Natasha calls the 21 Feet Rule. A demonstration is needed then, because Natasha insists that although the 'rule' has been tested over and over, it was never by a SHIELD agent, but she thinks her guard must be down because her hand has barely grasped the gun at her hip when Sif has her pinned against the wall, one of Natasha's own knives gently pressing to the side of her neck.
Sif grins at her rather smugly until the agent's training kicks in.
Tony, Thor and Steve find them like that – Sif flat on her back, Natasha straddling her with a blade hanging loosely between her fingers. Steve blushes while Thor congratulates Natasha on her victory. A knife misses Tony's head by half an inch when he says the women are just a little too kinky for him