When you're in love, nothing's better than being together! There's a whole list of stuff the woman could be doing, but the speaker knows that none of those things could be better than being together.
Think of all the time you are not Wasting and would not care to waste, (13-14)
This is in the same vein as "isn't this better?" Our speaker thinks that any time they're not together is simply a waste. The only worthwhile moments are the ones they spend with each other. Aww.
You who had rather lie in bed and kiss (19)
It seems this couple is truly in love, and puts kissing high on their list of priorities—forget going to work, doing the laundry, or any of that other day-to-day stuff. Kissing is numero uno.
Time flies, and I need not rehearse The rosebuds-theme of centuries of verse. (22-23)
Time flies when you're having fun… and when you're in love. The speaker thinks there's no need to recite old love poems to his girlfriend, and that their morning together has been testament enough to how much they care for each other.
If you must go (24)
Wilbur emphasizes "must" in this line to show how much the speaker wants to delay his lover's departure. He's really attached to her.
Wait for a while, then slip downstairs (25)
This line continues from the previous—in other words, if she really has to go, then he doesn't want her to go yet, and when she does leave, he doesn't want her to go far!
And bring us up some chilled white wine, (26)
Let the morning love session turn into a romantic picnic! It's as if the speaker is saying, "Don't leave. Just bring us something to snack on so we can stay here all day."