Sex and Breastfeeding
A: The breasts do not need to be off-limits at all, but the woman may want them to be off-limits because she is experiencing breast tenderness. However, her partner fondling her breasts will not hurt her in any way or affect milk production. Also, when a woman is producing milk, she will often eject a little milk during an orgasm or sexual arousal or stimulation, and that is fine—it doesn't hurt anything.
— Dr. Laura Berman