Answer: "Does passion just cool the longer we're together?": Yes.
"Is that normal, and do I just have unrealistic expectations?": Yes and yes.
"Would I be giving up something great (a life with this woman I love and respect) just to see what it's like out there?": Yes, apparently.
I can only take your word for it that you've grown together and you still love her. If indeed that describes life with your wife, though, then you've got something enviable.
Now, "enviable" doesn't mean much if you don't want it yourself; what other people want is irrelevant. However, the image of the lifelong, passionate romance is largely unrealistic. The only reason I won't say it's completely unrealistic is that the occasional couple do appear to look at each other "that way" decades into their journey.
But even then, no one knows what goes on behind closed doors, and no one knows what prices have been paid to get to that point.
If anything, the hazard of early marriage in your case is that you never watched the passion leave several relationships - or, alternately, never watched a relationship stay passionate but also volatile from beginning to end. These can be informative and persuasive experiences to bring to a marriage.
The only answer I can give you is speculation: that you won't find X, that you'll regret Y, that you'll grow to appreciate Z - so in the end all I can do is give you a few thinking points:
(1) No one is immune to what you're feeling, not even those who wait.
(2) Chances are, upon closer inspection, the lives of others will persuade you that your life is the only one you want. Pay close attention to what others around you have, and be aware of when you're projecting vs. witnessing.
(3) A reliable remedy for the "what-ifs" that won't risk terrible harm to your best friend: Keep your life interesting and exciting in other ways. Life offers so many things to feel passionately about, from sports to wine to the earth itself; every victim of boredom is a pointless one.
(4) Passion isn't just there or not there; it can be nurtured through attention. Maybe not at its initial intensity, but at a fulfilling one nonetheless.
E-mail Carolyn Hax at firstname.lastname@example.org.