I've done the "de jure open, de facto exclusive" thing before and you know what? It's GREAT. It reduces the stress of fights; you can date others while things are iffy, then dump them when they patch up. And it solves the LDR question, since you can acknowledge your love across distances while simultaneously dating (and enjoying the company of) locals.

It also creates an end-run around the sense of entitlement that women (and yes men) can pick up in relationships. Being de jure open required an acknowledgement of the self-determinacy of each others' lives, and can often head off so many of the petty BS that goes on in "traditional" relationships (i.e., you can't have guys night/girls night, you're spending too much time doing such and such, etc)