Flat-calling middle set

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MP ($117.50)
Hero ($215)
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SB ($202.80)

Preflop: Hero is CO with 4[img]/images/graemlins/diamond.gif[/img], 4[img]/images/graemlins/club.gif[/img].
<font color="#666666">1 fold</font>, <font color="#CC3333">MP raises to $4</font>, Hero calls $4, <font color="#666666">1 fold</font>, SB calls $3, <font color="#666666">1 fold</font>.

Flop: ($14) 6[img]/images/graemlins/heart.gif[/img], 4[img]/images/graemlins/spade.gif[/img], 3[img]/images/graemlins/diamond.gif[/img] <font color="#0000FF">(3 players)</font>
SB checks, <font color="#CC3333">MP bets $7</font>, <font color="#CC3333">Hero raises to $21</font>, SB calls $21, MP folds.

Turn: ($63) 8[img]/images/graemlins/heart.gif[/img] <font color="#0000FF">(2 players)</font>
SB checks, Hero checks.

River: ($63) J[img]/images/graemlins/spade.gif[/img] <font color="#0000FF">(2 players)</font>
<font color="#CC3333">SB bets $33</font>, Hero calls $33.

Final Pot: $129

SB was 26/6/1.3
MP was 34/9/3.5

okay so my flop raise was probably a bit too small, I didn't realize at the time that it was only a half pot raise

SB's cold call screamed monster to me, the only problem was I too had a monster. So on the turn I ponder the chances of him having a 5 (without it being a straight already), and I think they are small, so I'm not too worried about a draw. I would hate to get check-raised on the turn by 75 and miss my chance to fill up for free, so I check. (weak-tight much?)

Then on the river I don't know whether to raise or call, I think the only hands I beat that he calls my river raise is with 2 pair or 33

33 is just as likely as 66, so I guess that cancels out, but 75/52 feels more likely than two pair. I would expect a flopped straight more likely to be slowplayed then two pair.

So, does anyone like my justification of my weak-tightness?