Of time, by woman's reckoning, You've saved, and so may spend on this
The speaker thinks the woman has saved time by not doing all the things he lists in the first part of the poem.
A "woman's reckoning" might mean things that are important from a woman's standpoint. This might be offensive to people who think activities shouldn't be separated by gender (shopping, gardening, and lunching), but it might also be completely harmless (reading, training a dog, and attending college). We'll leave it as something for you to form your own opinions about.
The last word of line 18, "this" hints again at what they're up to, but doesn't give us anything concrete. C'mon, Wilbur, we're waiting!
You who had rather lie in bed and kiss Than anything.
Busted! They've been kissing in bed all this time. We had suspected something romantic was going on, and now our suspicions have been confirmed.
So this woman (and presumably our speaker too) would rather lie in bed together than do just about anything else.
This poem is taking a very sweet turn! It's almost as if Wilbur is arguing that nothing is more important than spending time with the one you love. Aww.