I am going to try and provide a more general insight to your question.

Although your question is based specifically around why we, as human beings, reproduce through sexual activity, I think we need to go a bit more general.

Our existence on earth is ultimately action based even though some of us know that we do most of the work vibrationally (through moulding our thoughts and state of being) first and we are then ultimately inspired to take a certain action in order to have a particular experience.

This is because we are physical beings having experiences in this three dimensional physical reality. This is how we have been designed to have experiences on earth which is different to how the other dimensional beings have been designed to have their experiences.

We have chosen to have action based experiences in this physical reality such as having sex to reproduce or eating to satisfy our hunger. However, although the examples here are basic needs, let us give some thought to other things in our lives. We do not just decide we need some food in the house and it just manifests. We actually have physically go to the shop and get it.

Ultimately, it seems that a physical action is required in order to have an experience of some kind and/or manifest something.

I have to look at this logically, and because I am human, in human terms.

Humans say that something is "alive" if it eats, reproduces itself, respires, etc. If aliens do not reproduce, then they are not alive. We already can reproduce without actual sexual intercourse, via in-vitro fertilization, but the baby still has to gestate within the human body. I am sure that someday, we will conquer this "problem" as well, and have baby-growing chambers (a la Matrix and other science fiction stories).

Some lower forms of life reproduce by "splitting" themselves- making an exact copy of themselves- like amoebas, but that is still reproduction, albeit asexual...Perhaps aliens can do this; I do not know.

Life reproduces itself- that is the human definition of life....

Peace,

Jai

it may be that stage in our development
where the archetype of manhood and
that of womanhood need to be fully known

when the reason for gender being
has no longer a purpose
humankind will be androgynous

That's a good question. While I'm not an expert in this field, I believe that it's because we're mammals. While a lot of aquatic animals can reproduce by asexual-ism or spore-ism, most land animals do, in fact, require sexual intercourse. As evolution goes by, males and females are slowly becoming single beings. Men are starting to lactate from their breasts (or pecs, whatever floats your boat), and females are being born with more and more testosterone throughout each generation. It won't be in the near future, but eventually we will evolve into asexuals (meaning we can reproduce with ourselves) in the distance future. I estimate by the year 3000 (If the earth is still around by then) your distant relatives will be hermaphrodites.