The theme of the poem "Musee des Beaux Arts" by Fleur Adcock is decay, and how that idea can apply to all things which be said to have "lived" in any sense, not only live creatures and plants but also relationships. She is addressing an ex-partner and asking for space, not only space for time as well. Like a dead bird, the idea of the dead relationship is ugly at first, too raw with emotional pain and hurt to even contemplate. We get the feeling the ex-partner is moving in again a little too close, a little too soon - perhaps wanting the best of worlds, freedom but not loneliness or isolation from the writer! The poet is telling him that one day she may be able to look at the relationship as coldly as a museum exhibit, admiring the "bones" of it for what they once carried.

Okay, the main thing to understand here is what the poem is about, which is indicated by the title.  The speaker is speaking to a "discarded lover" telling them (we don't know the sex of either speaker of discarded lover) what they should do.

The speaker is saying that new, raw, emotions are disgusting and revolting.  This is just like a newly dead bird.  Once the bird has been dead awhile and rots all away, it's clean and not repulsive anymore.

The speaker is telling the discarded lover to go away and not talk to the speaker, not ask for any emotional support or anything, until the emotions are more like the clean white bones of the bird that has been dead for a long time.