[El Gato]

If it is assault for a man to touch a dancer's breasts, then why isn't it entrapment for the dancer to be sitting in the man's lap, when he touches her breast?

A matter of consent. He's consented to have her sit on his lap, she has not consented, in fact it's most likely agains the house rules, for him to grab her breast. The same difference that turns sexual intercourse into rape.

[Pan_Yans Wife]

But to me, the circumstances are really questionable.

On the other hand, from my experience as a lactating mother, little or no stimulus need be provided for the "let down" reflex to kick in. And a shower can develop, without any warning.