I don't think it presumes a lack of free will. I think it presumes that a very small percentage of young people, in certain situations and with certain preconditioning, will tend toward activities that result in pregnancy. People can control themselves. Those young women that became pregnant, they controlled themselves in such a way that they became pregnant. Was that decision made easier by the stimulus they experienced? Well, it looks like in about 2-4% of them, the answer is yes.
One thing you need to remember is that you, and I, and everyone alive on this planet for that matter, comes from ancestors who all did one thing right, without fail, 100% of the time, each and every generation, going all the way back to the very first humans ever on this planet. That singular thing that every single one of your ancestors have in common? They procreated successfully and created offspring that procreated successfully.
Think about that for a moment and let's compare. What if every ancestor of a certain person from the beginning of human history was a murderer. How surprised would you be when that person committed a murder? Murder is, admittedly, a more complex behavior than sex so it is easy to argue that sex would be even more likely.
Also, it's not like there aren't tons of papers, experiments, and conjecture on the psycho-sexual motivations of humans. I have read that just about every human behavior has been related to or attributed to sexual drives by psychologist, psychiatrists, philosophers, and pundits. Everything from the drive to work and earn money and upwardly mobile social movement to speech patterns, vocabulary choice, and clothing. So it's not about whether or not people have free will, its about how people choose to use that free will, and how the world occurs to them when they make choices. Its about how people choose to have sex, when they will choose to have sex, and even why they choose to have sex.
I also seemed to notice a slippery slope in your writing that was coupled with a pejorative view of sex. If that is the case I fear we may have difficulty discussing this subject without knowing each other better. When discussing topics like sex there can be heavy filters and unspoken assumptions that lead to misunderstandings between people that are not aware of each others presuppositions. My goal in replying is not to say you are wrong, not is it to argue, but merely to say that any discussion of free will and sex needs to include the possibility that people will choose freely to have sex, and that those free choices are influenced by antecedents, stimuli, and experiences. It is true that acknowledging that people can control themselves is a good first step toward controlling that behavior. It is also just as important to realize what you are dealing with when considering human sexual activity. To deny that there are incredibly powerful underlying components to the human makeup that can be manipulated to increase the likelihood of procreation is shortsighted and leads to useless therapies like teaching abstinence, non-communication on sexual matters between children and parents, and ultimately sending children out into the world drastically under prepared and unsupported.
As humans we are, at our basest nature, violent sexual beings that don't always make choices that fit with the predominant behavior patterns we display to the world on a daily basis. Forgetting this leads to all sorts of problems. Like assuming that people will, with he proper training and teaching, decide to not rape an unconscious woman when she presents herself as such. I would love to live in a world where everyone could be taught proper self control and willpower, and would use those skills without fail. However, this is not the world we live in, and this is not who humans are. Drop a fully sedated and unconscious supermodel in front of 10000 young intoxicated men and regardless of how well you train them, some of them will rape her. Its not surprising, really. Remember those ancestors that I mentioned earlier, the ones that have been batting 1000 on the procreation front? Many of them were rapists, too.