Preamble

[ellajam]

This appeals to me, the sadness of what is a common situation shows through in the poem's bluntness. I think the contrast of the rich language at the beginning slipping into banality mostly works, it could use a little polish, some notes below:

Ten shins were bundled tight
beneath the knotty alder table
supporting our glasses.

I like the opening line, it caught my interest. My first image was splints of wood, when I checked to see if shins is used that way I came across this:
"Shin: noun, Buddhism.
1.
a Japanese offshoot of the Pure Land Sect, requiring only faith in Amida for salvation through his accumulated merit."
It was fun trying to apply that to the poem, though I'm sure it's off track. I'm sorry I read your comment about tensions because I don't know if I would have got it on my own, I didn't on my half dozen reads. So by L2 I have the image and I like it, it has the tightness and no corner to hide, the close face to face with no choice that sets the scene. I like bringing up drinking in L3, it foretells the future, but "supporting" is clunky.

It was Dean’s cabin,
and he sat at the table’s head I think "Dean sat at the table's head" would do fine.
drinking whiskey like a scoundrel
getting ready for a crime.

I drank slowly, measuring the eyes
of the girl opposite me, watching the hands
of the grandfather clock as it neared its chime.
ut oh, I was in a boat cabin, the tightest place I could think of, until the grandfather clock. Unless the size of the clock or grandfather has some meaning I'm missing you may want to leave that detail out and let "cabin" and those bundled shins work their image freely.  Other than my own trip, lovely lines. 

The girl’s father had been in the army, This line has no charm and I think it is too early to bring in the flatness that develops.
and she would move from base to base
like a leadoff hitter headed home.
Love these two lines, fun wordplay and gives a seesaw of wandering/goal focused with a fulcrum of sex.

She had met Dean at a bar a few weeks ago, Another clunky line, "Dean told me..." would be fine.
and he told me she was an angry fuck,
biting and scratching until she came.
Boy talk, great clarity into the big picture.

She brought two friends,
a slim blonde with crooked teeth,
and a short brunette with grey eyes.
I'm not sure you need need these three lines, you could say "Her friends looked like college girls" below and get the same boredom. They do flatten out the poem in a way I think you intend, maybe it would work better if you changed the flattened lines in the previous strophes, like falling off a cliff here instead of a gradual descent. I'm not sure.

They looked like college girls,
draped in cardigans with their thighs I like the break on thighs but the line itself is odd, maybe
no comma after "girls", then "draped in cardigans, their thighs".
wrapped in neon jeans.

I pretended to be interested
in their stories.
Really? I doubt it, the N's mind is busy with Dean's girl, the contrast between her and the others is clear. I'd like a stronger statement of disinterest or acceptance or settling, whatever you are going for here.

I hope this helps a bit.