What does the Nurse warn Romeo about in lines 155-160?

I'm not sure if our line numbers match but I think you mean when the Nurse warns Romeo not to lead Juliet into a fools paradise only to dump her in the end.

But first let me tell ye, if ye should lead her into a fool’s paradise, as they say, it were a very gross kind of behavior, as they say, it were a very gross kind of behavior, as they say.