River play with 2nd nut straight - reraise surprising fish raise?

[A_K]

This hand came up during a mutual swetting session with tyler_cracker. (Highly recommended, by the way; tyler had some great suggestions about my game; head over to the coaching thread or drop me a PM if you're interested in a session or two).

Anyway, both villans are lose-passive fish (roughly 50/4/.8). No specific reads unfortunately. I'm unsure of the correct line on the river here. At the time, I was startled by MP2's river raise, so I just called. (Thought he might be slowplaying a straight and was willing to push when the FD missed). Tyler rightly accused me of MUBS, and asked why I didn't three bet.

On further reflection, I think my call may have been OK. Assuming that MP1 will call one more bet but not two, a call gets us the same number of bets as a raise if we're ahead, but costs us fewer bets if we're behind. Thoughts?

Party Poker 0.50/1.00 Hold'em <font color="#0000FF">(10 handed)</font> link

Preflop: Hero is BB with 8[img]/images/graemlins/diamond.gif[/img], J[img]/images/graemlins/diamond.gif[/img].
<font color="#666666">3 folds</font>, MP1 calls, MP2 calls, <font color="#666666">3 folds</font>, SB completes, Hero checks.

Flop: (4.00 SB) T[img]/images/graemlins/diamond.gif[/img], 9[img]/images/graemlins/heart.gif[/img], Q[img]/images/graemlins/heart.gif[/img] <font color="#0000FF">(4 players)</font>
SB checks, <font color="#CC3333">Hero bets</font>, MP1 calls, <font color="#CC3333">MP2 raises</font>, SB folds, <font color="#CC3333">Hero 3-bets</font>, MP1 calls, MP2 calls.

Turn: (6.50 BB) 4[img]/images/graemlins/spade.gif[/img] <font color="#0000FF">(3 players)</font>
<font color="#CC3333">Hero bets</font>, MP1 calls, MP2 calls.

River: (9.50 BB) 3[img]/images/graemlins/spade.gif[/img] <font color="#0000FF">(3 players)</font>
<font color="#CC3333">Hero bets</font>, MP1 calls, <font color="#CC3333">MP2 raises</font>, Hero . . .

[edited to fix spelling of post title]