According to cultural anthropologist Ernest Becker, sex is such a problem because it reminds humans of their basic, core animal nature. Basically, humans deal with mortality concerns by embedding themselves in cultural symbolism (beliefs, values) or immortality beliefs (Heaven). Animalistic, physical behaviors threaten the very things we use to confront our mortality fears. Put somewhat differently, Otto Rank argued that procreation via such a phsical act is a direct threat to the idea that we are spiritual beings. Why would a spiritual being emerge from such a physical act?
Jamie Goldenberg, an associate professor of psychology at the University of South Florida, and a variety of colleagues, have empirically tested the relationship between mortality concerns and sex concerns in a variety of studies. The basic finding is that when people have been thinking about their own death (especially high nuerotic individuals), they show less interest in the physical aspects of sex (e.g., skin rubbing against my skin; having an orgasm; feeling my genitals sexually).
Other studies found that thoughts of physical sex acts increased the accessibility of death related words. So for instance, when given the word fragment, "g r a _ _ ", when reminded of the physical acts of sex, a person is more likely to complete it with "grave" instead of "grape." And, further, when the sex act was framed as romantic and loving (which gives it a human, spiritual quality. Animals aren't perceived as romantic), this eliminated these effects.
So why is sex so taboo?
Perhaps, we wine and dine, and experience love, as methods to procreate without being riddled with our most basic fear: death.
Goldenberg, J.L., Pyszczynski, T., et al. (1999). Death, sex love and nueroticism: Why is sex such a problem? Journal of Personality and Social Psychology,77, 1173-1187.
Goldenberg, J.L., Cox, C.R., et al. (2002). Understanding human ambivalence about sex: The effects of stripping death of meaning. Journal of Sex Research, 39,