She could, but she chooses not to. Any of the "could" phrases indicate that the person has, or has made, a choice.
Or rising in an elevator-cage Toward Ladies' Apparel. (3-4)
This line continues from the previous "could." It's an option too, but not the one the woman chooses. Why go to the mall when you could make out instead?
You could be planting a raucous bed Of salvia (5-6)
If she chose to, she could be gardening, but she'd rather be in a raucous bed with her beau.
Or lunching through a screed of someone's loves (7)
She could also be having lunch with a friend, but (you're probably picking up on the pattern) she's not. She's more concerned with her own love.
Or making an unhappy setter heel (9-10)
Another possibility: dog training. It doesn't seem quite as appealing as a romantic morning, and she chooses the latter. Well done!
[…] or listening to a bleak Lecture on Schoenberg's serial technique. (10-11)
Listening to a lecture on an Austrian musical composer doesn't really seem to compare to romance, and the woman agrees; she chooses a morning of love instead.
Isn't this better? (12)
"Better" indicates that the woman made a choice. Better than what? Well, better than all of the options listed before. Love, spending time with a lover, is a lot better than any of these mundane things, and she chooses accordingly.
You who had rather lie in bed and kiss (19)
"Rather" again indicates that there's a choice. She would rather lie in bed and kiss than do any of the aforementioned things.