One litmus test is to ask whether something would be appropriate behavior in a “normal” professional job.
I disagree -- the theatre world is not a "normal" world. We can't expect everything to run as a corporate office does -- it's just not going to. Our community is so small and so specific -- people are bound to fall in love and form relationships and get married and have kids. And because the world is so small, they're bound to come upon opportunities to work together again that they may not be able to turn down because they need the money/the work/whatever. Plus (IME) people outside the business don't always get it - the late nights, crazy schedules, crazy people...it can make a non-theatre relationship difficult if this is what you do for a living.
Maybe it's because I work in a company where practically everyone is married to each other. Currently we have two married couples within the company, another couple is getting married next summer, two other dancers are dating, one dancer is dating our lighting designer, my props mistress and my TD are married...it kind of goes on. A couple years ago I hired the guy I was dating at the time to be my ASM because I was in a bind -- it was great! I knew exactly how he worked and he knew how I worked and I wish I could have used him as an ASM more.
I guess I would say its not necessarily a good idea to try to start something MID-rehearsal process or run. But if you are already in a relationship with somebody and the opportunity comes up to work together (as long as the company doesn't have a specific policy on this) and the couple knows they can be professional about it, then why not grab the opportunity? I've always loved the chances I had to work with my significant others on shows.
(But that's just me...everybody's different and each relationship is different and should be taken on a case by case basis!)