In the old Order, when a Jedi in a relationship fell to the Dark Side, in most cases it wasn't because that loved one died, it was because they had already started down that path long before. Keeping it a secret to avoid the Masters finding out, telling all the lies and stories, etc. Deciet is the way of the Sith and the Dark Side, so its no wonder that a Jedi would easily fall to the Dark Side after getting into a relationship.
That being said, there were some, as previously mentioned that did it and succeeded without sucumbing. Corran Horn's father was a Jedi and died without falling to the Dark Side,. Just had to throw that example out there along with other examples already given.
Luke saw that you couldn't control the way you felt about someone, so rather than force a Jedi to conceal those feelings, he chose to allow it, so that it would be open, and then those Jedi who did have issues would have the support of all Jedi, and not have to suffer their pain on their own. A much wiser path in my opinion.