Yeah, me neither! But it kind of makes sense, in a fucked-up CLAMP way. (I SO can imagine Ohkawa reasoning: they both loved Watanuki the most, if they were with someone else and that person loved them the most they would hurt them; if they were with someone else who loved someone who wasn't Watanuki the marriage could get ugly; so they have to got together because then they love the same person the most and they both know it! Or something.)