It would have been nice if they had sex. I think, despite his personality and who he is, Doris would have reminded him that he was also part human. Kikuchi writes D as some celestial force, untouchable by man and beast, and perhaps to see him at a baser level would have been good in order to relate to.
As it didn't happen, I was quite pleased that they were able to kiss. Whether she knew it or not, Doris was able to make D feel human for that short momemt.
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