They can't conceive of devotion or friendship without sex, or even without sexual attraction. To them it's just inconceivable that Ruth loved Naomi as a mother.
Since Ruth married Naomi's son, and then Naomi's cousin Boaz, where do they see a "committed" lesbian relationship?
Same as when they presume that because David and Jonathan loved each other more than they loved women, that means they were lovers. For heaven's sake, how cliche is it that men never let their girlfriends or their wives interfere with their brotherly friendships with their buddies?
In fact, I find it difficult to believe a sizable percentage of the members of the MCC really believe that the Bible supports the idea that David and Jonathan were homosexuals -- it's just that they feel they MUST believe it in order to feel they are acceptable to God without correcting their behavior.
Here's a good reference for scholarly refutation of the MCC and other "Christian" groups' homosexualization of Bible doctrine: Responding to Pro-Gay Theology, written by Joe Dallas, formerly of the MCC. Dallas is loaded for best shots the Gay-stapo can take, which is why you likely have not heard of him; the media stay away from him.