You don't owe every random person on the website anything. I'd say I ignored at least half the messages I received from people. You also shouldn't feel bad for skipping over almost everyone based upon a thumbnail. The reason I said "prototype" males get the best results is because women use greater search criteria than men tend to use. They will filter by height, they will filter by employment, they will filter by hair color. They tend to be far more picky than men on the sites. So if you don't meet the cut of being 5'10"+, white, employed, college educated chances are you're getting far, far less hits back from the search results than someone who meets those criterion.
The only thing I ever searched for was Zip code (+/- 25 miles), age and body type. My girlfriend said she routinely searched by all those, plus height, plus employment, plus degree. From talking to other females that seems fairly common.