In addition to what sam said, it's noteworthy to know that what is usually meant by "sex equates the beginning of death" really is an old meme stemming from a simplification.
That simplification is : asexual organisms (especially unicellular ones) can go on dividing forever without ever "dying", sexual organisms are born, mature, may or may not reproduce, and regardless of that, eventually die. So as soon as sex appears, so does death.
As you said, even asexual organisms die. Even monocellular ones, such as bacterias, age. On the other hand, some multicellular ones appear to be non senescent, some trees for instance, have been going on for millenia.
At any rate, you could as well say that what kills us and any complex organism is just that, our complexity. It just can't be maintained as easily as a tree, a fungus, let alone a bacteria.
That being said, there's indeed a slight tradeoff between reproduction and repair mechanisms. I've often wondered about it, though, and it isn't clear where you should draw the line as to whether or not you have been "reproducing" and therefore are more expendable, as viewed through an evolutionary lense.
In other words, when does your body "knows" that it has been reproducing ? Is it when your testes start maturing, producing certain hormones such as testosterone ? Is it when you perform a sexual act ? If so, must you be with someone else, or alone ? Must it be repeated many times, or is once, or a few times enough to clue your body ? Is it when you are in the continuous, or temporary presence of a potential, or declared sexual partner ? Maybe is it when you are in the continual, or even temporary presence of children ? And must they then be your own to have an effect on your rate of aging ?
In every one of these cases, how do your body "tells" the difference ? The difference between lone sexual activity, or with a partner ? The difference between being part of a group with members of the other sex, and being part of a couple with one ? The difference between being part of a group containing children, and having a family ?
It is evolutionarily relevant for the body to know how to make such a difference in each case, if there's a tradeoff between sexuality and longevity.
But not all of them may be amenable to providing the necessary clues for you body, for instance, it may be difficult to tell the difference between your own offspring and those of others in the old times (a touchy topic, and for which we have been fine tuned to make sure they are; besides inclusive fitness has it that in small groups of extended family, your uncle's children are a bit like your children, so to speak, from the gene's point of view).
And yet also, not all of these clues may refer to situations that have been remaining the same, for instance, it may be that we masturbate more nowadays than in the past (not terribly sure about that one though), and if it is the case, it would mean that a signal that was a stable measure of the probability of having children (performing sexual acts, the body wouldn't care to pay attention, if that was even possible, as to whether or not it was with someone else, since reliably almost always was), is not efficient anymore.
Regardless, sexuality, bonds, coupling, parenting, all initiate noticeable psychological and physiological changes, so it is almost certain that the body has certain ways of assessing whether or not it has been reproducing, or at least, if there's a good chance it has.
Any particular phase of your life require different strategies in order to keep your reproductive fitness high, as for instance, growing into an adult first, then switching modes and starting to invest energy into searching a mate and seducing it, then switching once again once you beget children, to care for them and make sure they survive until at least they are fit and grown enough to care for themselves. At which point you might still be good to go and make some more children, but it is (you are) not necessary anymore, as your genes have been passed over on to the next generation.
Edited by vyntager, 09 April 2008 - 01:20 AM.