"That restaurant is romantic."
It's correct grammar to use很。 without 很 it means a comparison to another specific place, in this sentence it would be like saying "more romantic" than somewhere else. The connotation is bad, in many cases it can make the speaker sound arrogant or boastful. 很 in most cases isn't really a huge emphasis. Often native speakers will use other markers of degree such as 十分 or 非常 to indicate a higher degree of something. In short, the translation of 很 is optional because grammar requires it's presence
I was always taught 很 is only necessary grammatically with single-character adjectives. As 浪慢 is a two-character adjective, it does not need to be accompanied by 很. Your argument is connotative rather than grammatical. Grammar does not require 很 to be there as 那家饭馆浪漫 is grammatically correct but awkward to native speakers.