In this poem, the first quatrain opens by saying that people who are in love should get married. Then it goes no to start talking about what love really is. It says that love does not really change according to the circumstances.
In the second quatrain, the speaker continues in this theme. He says that love is like a lighthouse that does not move. He also says that love is not something that you can put a value on -- it is beyond that.
In the last quatrain, the speaker finishes up by saying that love does not change over time.
In the couplet, he says that if he is wrong about this stuff, he will take back everything he has ever written and he will believe that no man has ever loved anyone.