Our left and right sides are mirror images, but our front and back and top and bottom are not.

Nivens' (Or was it Pournelle's?) "Moties" were not bilaterally symmetrical. Bisexuality is common, but there are many other alternatives, even on earth. There are hermaphrodites, asexual reproduction, sexual alternation, or specialized sexual castes (as in social insects). Even among higher mammals with conventional sexual/reproductive equipment, sometimes reproduction is restricted to the alpha male and female (as in wolves). The only truly common and universal sexual strategy I can think of is that there are never MORE than two sexes. More than two probably interferes with the statistics of natural selection.

I suspect we may find alien sexual practices quite disgusting (and vice-versa). Then again, they may use a sexual prosthesis and have developed some technologically-assisted form of reproduction.

Pierson's Puppeteers have a third sex, but it is not really a sex at all. It is a different (non-sentient) species altogether which they have domesticated and in which they deposit their eggs and upon which the larvae feed after hatching. A lot of insects on earth do this.

Like I said. We should be ready for anything.