I think dancing fool is right. He might just be using you as a back up. He'll only write if that other relationship doesn't work out. Its up to you if you are willing to be someones second choice.
When you think about it, unless one has never been in a previous relationship, they're someone's second choice (or third, or fourth...).
Given the odds of success of your first match, or even the fifth match being the 'one' what's the harm in leaving the match open?