At what point is ogling acceptable?

The fact is everyone who isn't asexual looks at atractive people (whatever it is they consider atractive). But if they don't have the decency or ability to control themselves as to not make it obvious how can you expect them to have more self control or decency when they are sure they are not seen? Personally I think you are harming no one by looking but you shouldn't if you have someone out of respect for that person, you might hurt their feelings or the other people who start saying "i bet he/she is cheating on that poor girl/guy you see how he/she is oggling people all the time?" might hurt their feelings by treating them as someone pitiful. In short have some respect, don't stare unless you're single.