This is a very touchy area. There are different ways to loook at it-some ok, some bad.
From someone who has been married 13 years, let me first say that this butterflies and raging chemistry thing is overated when thinking long term. NOBODY has this all the time. I would guess that Brad Pitt and Pam Anderson would have off times if they were together for awhile. This can fool some people that it's time to simply move on. I hate this. I hate when people, especially married people automatically use this as an exuse to cheat or fantasize incessently. Realize that with the other person you desire, the same thing would probably happen down the road.
That being said, there does need to be an attraction. Maybe many people think it's shallow and don't want to admit it, but it's true. Sadly, many people end up with people that they are not really attracted to. I have strongly claimed that many women are not attracted to their husbands-as Rainfall can attest to from the other thread I've been posting in. They marry for security, they feel he can be trusted more, they don't want to worry about other women,and money. Hence, all the male strip popularity and women's subsequent behavior during those shows. LAdies, yes I know men are guilty of this too, I just feel women are more guilty of it-and it's sad. Just think, the woman marries you for the reasons listed, but doesn't really want you physically, desires others, and it has to make you feel like s***.
I have a good example. My wife and I just took a 3 day cruise to the bahamas after thanksgiving-without kids

. I noticed that were plenty of couples where the guy was not near as equal in looks as a man as his girls looks were as a woman-if that makes sense. That is, I just think the equal to the girls looks would have been someone much better looking. Of course, I'm a guy judging other guys, but I think I have a good idea what women will like- at least the obvious ones. I know I'm not crazy here because A lot of other guys have aggreed with me. It actually gives them hope. I wonder, how are these women when they are away from the guy. Are they fantasizing a lot, are these the girls in the strip clubs?
Also, just look at life around you. Does any woman here think that the model who is married to Donald Trump is turned on by him when he takes his clothes off? he gets a pass because he has money.
So, I would say to the original poster, to try and figure out why you feel this way or don't feel this way. Maybe it's just that the honeymoon is over, but you can still add spark. Maybe those past passioante realtionships you mentioned would have ended up the same way.
Then there is the case of maybe you thinking you had better looking people in the past. This may be natural ,but it's also flawed because once you start thinking like that you forget that there will always be soemone better looking than you, and this includes you too. Then, with that reasoning the only people who should mate is ..well.. Brad Pitt and Pam Anderson, or whatever stars you want to choose, but you get the idea.
You have to try though, or you may regret it later. If there still is no passion, then sadly, I would end it. Like the other poster said, passion and sex are important. If your just not attracted to the person, while you may not cheat, you'll fantasize more. That's not a great thing, no matter what some may say.