I only feel jealous when I feel there's a threat to the relationship or some sort of shady, underhanded stuff going on behind my back and usually I'm right. But, I wouldn't let my partner know I'm jealous...No way. I just sit back, bide my time, observe and wait for evidence that inevitably appears telling me my gut feeling was right all along and then I let them know I know what's going on...does that make any sense?
The problem with many cheaters and liars is they do such a good job of convincing their partners they're the only one, they'd never lie and cheat, blah blah blah so that their partner thinks they're both on the same page until voila, their true colours come out! Sometimes this type of deception can go on for years...leaving devastation in it's path and trust broken...
Most people are not going to say up front that they're the type who will screw around on their partners...that's what makes them liars and cheats! If they are upfront and their partner can accept that in the relationship, good on them. If they can't I agree, it's their responsiblity to walk away....