"Together" specifies that "close" is referring to those two things. "McDonalds and KFC are close together" means they are next to each other. "McDonalds and KFC are close" means they are both within easy walking distance of where we are now.
However, it is different with people. "Mac and Kay are close" means those two people have a good friendship. "Mac and Kay are together" probably refers to a sexual relationship, but if you say "Mac and Kay are close together" you are back to saying they are next to each other, adjacent desks maybe.
I am struggling to make sense of the poetic insight the author has perceived in the situation depicted, and embedded in this phrase. I start by accepting that she has made an observation that she wants to communicate, so I take it that there is in fact a meaning here. I think this is important, because there really does not have to be any specific meaning beyond some evocative impressionistic observation that can't be communicated in words. It's also important for me to point out my process, because I want to emphasize that, at best, all I'm doing is making an educated guess as to the meaning, and only the author could have explained it precisely. The rest of us can only surmise.
And I think it means one of two things.
The first possibility is that "nerve" means simply "courage," and the necessity comes from either the fact that courage demands of a person that he or she act in a particular way, or the fact that these people feel the need to show their courage for others to see.
The second possibility is that "nerve" means what it does in the phrase "you've got some nerve." This use of nerve equates to "arrogant gall" or "impudence." If this is the meaning, I would then say that this kind of "nerve" carries its own necessity of expressing itself. If you're impudent, you act that way by your very nature.
The larger context of this quote, which would give us a better idea of the circumstances and the character of the people involved, would likely help us interpret it even better, but this is the best I can do with what we've got.
Best Answer
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 :)