"without using religion, how is sex spiritual?"
well your using a very limited definition of spirituality if you disallow "religion" which is the spirituality of at least 85% of the world.
i think its common sense that sex =/= just a physical act. its something emotional and feels as if its even deeper than that. in that sense its spiritual. compare the emotional functioning of a relationship before and after sex. unless ur really able 2 block your feelings out (porn stars who've gotten used 2 doing that maybe?) sex should always b emotional.
how emotional, and how much u allow that 2 b considered "spiritual" is up 2 u.
btw how do u define the spirit?