disclaimer : I'm defending a fishy play... But here goes :

The draw heaviness of the flop makes it likely that I also have the best made hand at the moment (esp since the non-spade card is a 5 ; if it were an A/K/Q it'd be much more likely that my 7 is behind).

So villain's likely range is (in order I'd guess at based on available cards and preflop action) :

-5sXs (x being likely A/K)

-10-X (protecting)

-555

-4s6s or 4x6y

-77 unlikely since I hold a 7 and 1010 unlikely since it usually raises preflop

I instinctively responded that I'd call with 7x here without thinking much. Now that I've actually thought it out it's not that fishy at all.

When a spade falls I can fold easily.

When I hit my 2 pair or my 7 I can expect hero to pay me off with 10x since my hand looks like busted spades.

Also, I can think hard about calling a bullet if, say, the turn checks through, the river's a blank and the flush misses.