In As You Like It, what does Touchstone mean when he says "for honesty coupled to beauty is to have honey a sauce to sugar"?

It is always important with such questions to look at the
quote in context and to consider what else is happening in this scene. This quote is
said in Act 3 scene 3, in which Touchstone courts Audrey. However, this "courting" is
really a parody of courtly love that has been displayed elsewhere in the play, for
Touchstone seems to accept Audrey's unfaithfulness and also admits openly that he is
only courting Audrey so that he can satisfy his lust. The quote you have identified is
said as a response to Audrey when she asks Touchstone if she would have her be honest.
Note the full response that Touchstone gives her:

readability="8">

No, truly, unless thou wert hard-favoured; for
honesty coupled to beauty is to have honey a sauce to
sugar.

If we look at this
response, we see that Touchstone is arguing that he would not have Audrey be "honest" or
faithful to him, because honesty linked together with beauty is like having honey with
sugar. Therefore Touchstone is saying that it would be too much of a good thing. We
therefore see in this scene that Touchstone "courts" Audrey in a way that is a parody of
courtly love. He is ruthlessly honest about his reasons for wanting to marry, and he is
likewise honest about his expectations of Audrey as well. The "love" that Touchstone and
Audrey share is more about satisfying their immediate physical desires rather than
anything else.