2/5, reasonable soft game at MGM. Limps around and I get a free ride in the big blind with 53 off. Board rolls off 762 rainbow. It checks through on the flop. I don't see the percentage in betting the sucker-gutter into 3 people. Turn is a 4, giving me the sucker straight and putting out a bavkdoor spade draw. I bet 15 into like 20, guy who seems a bit too aggressive pre-flop but otherwise standard who limped utg calls, everyone else folds. River rolls of an ace of spades, completing back-door flush draw. I bet 40 figuring I can get looked up by weird pairs that had straight draws, odd two/pairs etc. He raises to 110. I fold. Too weak? I just don't see even somewhat aggressive players bluffing a back door flush or turning some weird pair/busted draw into a bluff when I bet relatively large. Thoughts?