A vivid simile. いいよる (言い寄る) = 'to court, to make advances.' 味つけの入ってないスープ is a relative clause: 'soup that doesn't have seasoning in it.' をすする ('to slurp') evokes the act of trying to taste something disappointing. ~ようなものだ ('is like X') closes the simile. A humorous, sensual metaphor for unrequited flirtation.