partly because in your orange shirt you look like a better happier St. Sebastian (3)
Since he was a Roman soldier back in his day, we're guessing that St. Sebastian probably didn't have too many orange shirts hanging in his closet. In fact, most visuals depict him more like a shirtless pincushion than anything else. Still, our speaker sees this orange shirt as a visual marker of the addressee's joy and perfection. This you is even "better" than a saint. And all it took was an orange tee.
partly because of the fluorescent orange tulips around the birches (5)
This line is intriguing for a couple of reasons. First, who knew that light bulbs came in orange tulip form? We kid. Actually, this orange is a visual echo to the orange tee shirt that our speaker praises in line 3. Again, the color suggests a kind of joy, energy, and overall warmth to the scene that the speaker is describing. It's as though things take on a vibrant appearance as a result of the love he feels. Or do you think this is a case of his emotions coloring what he sees?
partly because of the secrecy our smiles take on before people and statuary (6)
Sometimes, love means that even your facial expressions become totally inscrutable to everyone but you and your honey.
and the portrait show seems to have no faces in it at all, just paint
you suddenly wonder why in the world anyone ever did them (11-12)
Appearance is not entirely positive in this poem. As he reflects on the joys of being in love, our speaker becomes disenchanted with the effect that visual art has on him. It's like the appearance of these fine works has just lost its mojo. Their appearance fundamentally changes, and they aren't wondrous works of beauty anymore. They just look like… paint.
at you and I would rather look at you than all the portraits in the world (13-14)
All together now: Awww. We don't know about you, but to us this compliment stands out as the most meaningful in this poem full of compliments. It's clear that the speaker is a visual arts nut, but he's now saying that the appearance of all of those portraits just can't hold a candle to the appearance of his beloved. If that won't get you brownie points, we don't know what will.
and what good does all the research of the Impressionists do them
when they never got the right person to stand near the tree when the sun sank (20-21)
Here is the speaker's fundamental gripe with visual arts. Despite their best efforts to represent the appearance of the world, artists like the Impressionists are missing the point. It's not about representing life. The goal is just to live it.