if you want to vote for moral absolutism then how do you determine the divide? You can't decouple such considerations from time as the passage of time is what marks a shift from when an individual can participate in "mutually gratifiying sex" and when that sex is exploitative. If an immutable line exists where is the zero width line between T and F?
I already explained where the divide lies -- between a mutually gratifying act and an exploitative act. Describe to me an "immoral" sexual encounter, in the context of the two (or more I guess) participants involved, that is inconsistent with the above.
Age comes into play only because the age of the younger participant, and the relative difference in age, can provide clues as to the likelihood exploitation has occurred. For instance, a prepubescent child cannot, biologically, derive pleasure from sex. Therefore, no adult who has sex with a prepubescent child could argue reasonably that the sex was "consensual" or healthy for the child.
However, that does not mean all sex between two postpubescents is by definition consensual or moral. An obvious example is forced rape. Answer me this: What on earth does age have to do with the immorality of rape? The rapist could be 15 and the victim 50, but so what?
We have made age relevant in our legal system because it's extremely difficult to prove intent in a court of law. It is possible, as some rape prosecutions are in fact successful. But those cases are also extremely painful and humiliating for the victims, who often describe the experience as akin to being raped a second time.
Therefore, if we set a legal age standard below which the likelihood of exploitative sex is far more likely, we can base the law around the age standard, make it an open-and-shut case and spare the victim hurtful accusations that she -- because it's almost always a female victim -- "asked for it" or was otherwise complicit in her own victimization.
But there's always that occasional case that doesn't fit the mold, such as the Letourneau case in which the "victim" was under the legal age of consent, but at the same time was clearly a willing participant who enjoyed the relationship. Was it immoral? It was certainly illegal, and from that standpoint alone some would say therefore immoral. Others still claim it was exploitation -- the kind in which the child "thinks he wants the sex" but is in actuality a victim. And to that I say, "Get real."