*Booping noses* means to rub together your noses as a sign of affection. 

And *sweet nothings* are the meaningless sweet words of affection and love  you'd say to your lover.

This sentence tries to say that if you are a couple who does that in a public place to the extent of making others uncomfortable, then probably you deserve to be ridiculed.

**Edit:**

@Inglish Teeture is right about the exact meaning of "you deserve to get breadcrumbs thrown at you". That meaning somehow didn't occur to me :)