They are common, but rare. It is an innate natural response to be jealous, it's all down to competition. Even if there is complete innocence on the boyfriend's part if friends with a girl, women will always have that slight element of jealousy because it's competition. Blokes are the same. We display it more because we're a demonstrable sex, we show more of our emotions.
I also think that blokes unecessarily make their girlfriends jealous, to make them feel in demand/validated, which is ridiculous. I have always been a jealous person, but the level of jealousy portrayed depends upon how secure my respective partner makes me feel. He may give me no reason at all to be jealous, with regular reassurance and validation, yet there'll be boyfriends who don't, and haven't matured enough to understand that women need to feel reassured and shown that they are the only girl for them.
I've had a relationship were an ex would make (what would be seen by him as funny), comments about other women, yet it would fuel my jealousy. I don't want to hear him make comments about other women (not necessarily celebs who are unattainable), but your regular girl on the street. My ex would also regularly stare at other girls, and make it obvious. So jealousy is a two way process, its about being secure within yourself, but is also the responsibility of the other person not to make one self feel insecure. Jealousy is a way of showing that we feel threatened by our competition, in effect, and blokes do feel the same it's just that they don't show it.
I've never made a partner feel jealous in any capacity. I want them to feel as though they are the only one I have eyes for. Of course I look at other men when with a partner, it's human nature, but the difference is I do it discreetly and admire for only literally a couple of seconds. However some blokes lack the tact or discrepancy when looking at other girls - they make it obvious, and sometimes go too far. BUT, if im with the type of guy who is funny by nature and rather than being seedy about it, does it in a comedy sort of way (like, 'ooh look at them boobies!!', yet backs it up with a very reassuring comment like 'but yours gorgeous are the best!', then it doesn't feel as threatening. But to ogle, and make his sexual desire so apparent, it's a horrible feeling.
If guys are friends with a girl, we will ALWAYS feel a degree of threat or jealousy - even if you reassure your girlfriend to the max. We are competitive creatures. It would take an AWFUL lot of convincing to show that my boyfriend is not remotely attracted to his female friend. If, she is even remotely pretty, we'll be slightly jealous. For me, being genuine friends with the opposite sex (i.e boy being friends with a girl) is only possible if neither of them find eachother attractive to any degree - then it can work. Ive stayed friends with an ex because i saw him more as a brother, than as a boyfriend. I didn't see him in that light in the end, and that's why i know its possible to be friends because i dont feel any attraction to him at all.
Nothing wrong with a small amount of jealousy, its a sign that they do actually give a **** that they feel threatned or fearful of losing you.
I knew gut wise, that the day i stopped being a little jealous all together with a boyfriend is when i wasn't in love with him anymore. That's from experience. If your girlfriend starts jealous, and then suddenly appears to not be jealous anymore, from my point of view, start worrying.
There are of course relationships were there is no jealousy, which is a situation whereby both of them give eachother the validation and reassurance on a consistent basis.
If you innocently, or deliberately (verbally, or physically) give your girlfriend a reason to be jealous then blame yourself. We are jealous most of the time because guys give us reason to be. Like i said, a small amount of jealousy is healthy and a good sign someone cares about losing you, but unhealthy jealousy is usually indicative that you're giving her reason to be.
I'm alot more relaxed now (im 31), than when i was in my young twenties, but i've learnt that guys sometimes lack the sensitivity or tact when it comes to admiring or being friends with the opposite sex. Just think how you would feel if the shoe was on the other foot.