I think you'd have to be routinely effed over in marriages, long term relationships etc.
Being turned down by a slew of girls that you never really had a chance or never really dated? I think you're just being overly dramatic.
I think if you find that right person your outlook can change pretty quickly.
I think people feel how they feel, and while you can guide them in positive directions in their life, I don't think you can really tell someone how they should or shouldn't feel about a particular situation.
And really, that goes for anything in life, not just whether or not you think someone has been spurned enough to feel jaded.
As far as the issue of boyfriends / girlfriends hanging out with the opposite sex?
I agree, it's all about trust. When I was with my ex-girlfriend, she had a male friend that she hung out with a lot. However, I never felt a single tinge of threat or jealousy. I'm not saying my girlfriend did this just to make me feel better or whatever, but she had me meet him, I got to know him, I saw first hand the dynamic between the 2 of them, and I knew that he was not a threat to me and that she wasn't a threat to go off and fool around with him or anything. I didn't even particularly think those things to begin with, as I know they were friends long before I came into the picture, but it was never a situation that I ever had to feel even the slightest bit of jealousy over.
On the flip side, despite my antics with participating in cheating, I seem to be a guy that guys trust their girlfriends around. I had one friend long ago when I still lived in California, and her and I were always spending time together - alone, while her boyfriend was away, even often times late at night at her apartment. Trust factor was that I was friends with her boyfriend as well, they both knew me, and I never had any kind of intentions of making a move on her, despite the fact that people always thought her and I were having an affair or had secret feelings for each other. She was physically attractive, sure, but I didn't find myself ever having any desire for her. She was the cliche "like a sister" to me, and the one time I never had a problem with a girl saying I was "like her little brother".
There's also a girl more recently that I went to school with who was in a serious relationship, and she told me that her boyfriend actually preferred when I was around her, because according to her, he felt that I would watch over her and take care of her when he wasn't around. Basically, there WAS a guy who was always making advances on her (even though he too had a girlfriend), and wouldn't take no for an answer, and more than once I would have to separate him from her and keep him away from her. So her boyfriend ended up trusting having me around, and in a similar situation, while the girl is physically attractive, sure, I never had any desire to make a move on her or pursue anything further than just being friends.
So I guess it is all about trust. Some people trust more than others, and some people are more trustworthy than others. Despite some of my questionable actions in the past, most people that know me still believe me to be a trustworthy person around their women, so I guess there's that.