I voted "I'm taken, so I DON'T get jealous" but to be honest: When I was single, I was quite jealous of people in relationships. Especially in spring, when all the lovey-dovey couples were polluting the world outside my window. XD
I'm in the middle really, as I see some relationships and sometimes just get the feeling of damnit, I wish I could do so and so with someone, not anyone in particular, so I'm not really jealous of them, or anyone at all, I just feel a bit meh *must resist temptation to ramble for the second time in 10 minutes*
Sorry when you say jealous, do you mean as in... You being attracted to a person who is in a relationship and wishing you were their partner instead of whoever it actually is? Or just jealous of people in relationships?
I'm assuming the latter.
Personally I don't when I'm single, and obviously not when I'm in a relationship. But I understand the feeling. I guess it is somewhat distressing to see people happy if you're lonely. But on the other hand the grass is always greener on the other side. Not all relationships are "good" relationships, you might be aspiring to a thing that actually results in a negative experience for you. As you can be exposed to such fun things as being cheating on, fights, breakups and even physical/physiological abuse and all sorts of stressful things.
I think it's important to keep your dignity, being in a relationship doesn't "complete" you and pining for it makes you look needy.