If you simply assume that everything around you is just like the few things you have seen up close, you may not notice clues that some things are very different.   This applies to the sex lives of your neighbors, and to alien biology.  On the aliens, Paul Davies in the November Scientific American: 

Thirty years ago the prevailing view among biologists was that life resulted from a chemical fluke so improbable it would be unlikely to have happened twice in the observable universe. … In recent years, however, the mood has shifted dramatically. … Biochemist Christian de Duve called life … "almost bound to arise" on any Earth-like planet. …

How can scientists determine which view is correct? The most direct way is to seek evidence for life on another planet, such as Mars. … An easier test of biological determinism may be possible, however.  … If life does emerge readily under terrestrial conditions, then perhaps it formed many times on our home planet. To pursue this tantalizing possibility, scientists have begun searching deserts, lakes and caverns for evidence of "alien" life-forms-organisms that would differ fundamentally from all known living creatures because they arose independently. …

At first this idea might seem preposterous; if alien organisms thrived right under our noses (or even in our noses), would not scientists have discovered them already? It turns out that the answer is no. The vast majority of organisms are microbes, and it is almost impossible to tell what they are simply by looking at them through a microscope. … Researchers have classified only a tiny fraction of all observed microbes.

To be sure, all the organisms that have so far been studied in detail almost certainly descended from a common origin. … But the procedures that researchers use to analyze newly discovered organisms are deliberately customized to detect life as we know it. These techniques would fail to respond correctly to a different biochemistry. If shadow life is confined to the microbial realm, it is entirely possible that scientists have overlooked it.