An equilateral triangle is about as stable a shape as you can find, but even triangles suffer stresses. You are still in the two-honeysmoon phase and everyone is, I am sure, on his or her best behavior. This is certain not to last. Sooner or later someone will feel neglected or insufficiently supported and will not repress the urge to make that snappish comment, and somebody else will come back with a "Yeah? If you're so ____ing _____ why don't you ______instead of ______ing?" and somebody else will roll his or her eyes and somebody will yell at the eye-roller for eye-rolling. It is inevitable. And soothing three egos and salving three sets of hurt feelings is exponentially more complicated.
I would also not discount the lack of societal support for nontraditional unions as a source of yet more stress. I hesitate to draw a direct parallel to gay couples, but not having people beam at you when you announce your intentions and not having all the aunts tear up at the sight of the lovely bride (groom) and not having the chair dance and the "mazel tov and siman tov" can be a real loss. It isn't all about the health insurance and the tax breaks or even about commitment — marriage is also about societal support and approval, and that support and approval does help solidify a union. As I said, not exactly the same thing, though. Your situation is worse.
What? No. I don't disapprove. But other people will, so strenuously that you will feel obligated to keep it a secret. And while secrets can be sexy and sharing them can be bonding, living in hiding (or in a situation generally misunderstood or despised) is ultimately pretty destructive. Which is certainly no reason not to do it.
The complications of a multiple marriage (equivalent) go far beyond potential threats to peace on the home front like jealousy, possessiveness, and schedule difficulties. (Have you never watched Big Love? Even if you're not planning on founding your own splinter sect, you might want to.) Spouses are family, but what are second spouses? What happens if Jen has a family emergency, is ill herself, or otherwise in need of immediate succor? Your boss understands "My wife's mother died, I'll be out this week." She doesn't give a damn about your wife's girlfriend's mother. What if Jen wants to have a baby too? The three of you may fall easily into a family pattern that works for you, with one mommy and one mama, or whatever — children never seem to find unusual arrangements the least bit troublesome, since they have no idea what "usual" is and what's normal is what's normal for them. Schools and soccer coaches and other authorities, however, will have Opinions.
In other, fewer words, sure, you can do this. Jill can do it. Jen can do it. But living a life that sounds like somebody else's dirty joke is not going to be easy. If I'm sick of hearing "You've really got your hands full!" when I walk by with my twins — imagine how tired of it you're going to get. You might want to get in practice now: "How do you figure out which one to do first?" sniggers the office wit when you let slip that you're finding your home life complicated, "I wish I had your problems."
"Dude," you'll sigh, as you attempt to side-step him to get to the copier, "You have noidea." And he won't.