I have noticed that I am jealous of people who are not "secured". That is, tomorrow Seamus and his coworkers are going to a strip club. I have no problem whatsoever with Seamus going and doing whatever he wants. But his coworker I like? I feel a tinge of jealousy. I hate that I won't be there to see what happens. What if he meets someone (unlikely at a strip club) and happens to be mono (more likely)?
When I see him with females I wonder if he likes them, etc. I'm way more possessive and jealous, because since we're not together, his getting with someone else could prevent a relationship. On the other hand, I'm secure that new relationships won't take any partner from me.
I know this jealousy is irrational and the fruit of insecurities, so I try to keep it low, but it's not always easy.