"But your worries aren’t entirely irrational. While I agree that men and women can be just friends, proximity is also a huge factor in bringing couples together (“The One” mythology notwithstanding). And, yes, Boyfriend can be happy with you and still fall for her."
"Given all that, I think your best bet is just to say, “I think there’s an element of tempting fate with a female roommate, but objecting to it feels wrong, too,” and let him figure things out from there. Or don’t say anything and let the story play out. That puts us right back where we started, but that’s not always the worst place to be."
https://www.washingtonpost.com/lifestyle/style/2011/09/28/gIQAch7jYL_story.html