Quick And Easy.

He chose to use pleasure because many people can still concentrate and fight back while feeling pain. In fact, their fight or flight instincts would be engaged because of the pain. 

Pleasure on the other hand is good for the body. It is not a threat that will make them resist. In fact, the body will crave more of it. Most importantly, very few people can concentrate on anything when they are feeling intense pleasure. 

His move caused their eyes to roll back in their heads. One of them fell to the ground, shaking with euphoria. The other only stumbled and felt dizzy. 

Meanwhile, he felt a sharp pain in his head. It is because he had used the ability on two people at a time. The pain was extra bad since Black Widows main attribute was her mind. She had a powerful mind, which granted her a powerful resistance to his ability.