This is off topic, but I'm just curious. If you've been together for 17 years now, why are you not married?Many reasons ;-)
* We're both atheists - so we don't think any god(s) are going to be annoyed
* We live in the UK so, at this point, living together without being married isn't frowned upon socially - not by anybody we care about anyway. There's also the slightly weird thing that at this point people just assume we are married unless the topic is brought up specifically.
* Neither of us are particular fans of getting up in front of large groups of people so the whole "wedding day" concept isn't much of an attraction. Quite the opposite if anything.
* No kids (by choice) so don't need to factor in that as an issue.
* The whole needing state/peer approval for a relationship seems slightly weird to both of us. Our commitment is personal and private and doesn't need any external validation thank-you-very-much.
* Personally we see staying together without marriage as a more positive statement about the state of our relationship. We're together because we want to be together - not because a divorce is too much effort.
The only hassles have been around stuff related to medical care for relatives and each other (a few idiots are a little too strict about the definitions of family and need paperwork), along with a mild tax hit since we don't get the married couples allowance. At some point soonish we'll have to sort out some of the legal stuff around investments/wills, etc. - but most of that's already been dealt with with.
If the UK's Civil Partnership arrangement was available to straight couples we might have gone for that - just to make the legal stuff simpler, but is isn't so we didn't.
TL;DR - Why should we? ;-)