And I absolutely disagree. The only thing that happens as a direct result of the sex act is that sperm are deposited in the vagina. The act does not put any sperm in direct contact with an ovum capable of being fertilized. It does not directly cause the formation of a zygote or the implantation of the blastocyst.
The sperm try to swim up to the ova. They may or may not make it and there may or may not be ova there. A sperm may touch an ovum, and if its chemical coating (I forget what chemical this is) is strong enough, the ovum will open and the small head part of the sperm will go in and the rest will be destroyed. Then, processes for the formation of the zygote occur. Then, the zygote divides (morula stage) and the morula, reaching a sufficient number of divisions, reaches the blastocyst stage. When the blastocyst cannot survive on internal resources and scavenging, it implants (ideally) in the uterine wall.
The vast majority of zygotes, morulae, and blastocysts do not reach successful implantation (some say up to 75% of these and early implanted embryos do not implant or stay implanted). There are many reasons why. One is that the endometrial tissue seems to screen out some blastocysts right away. However, there may also be too slow a development of the placenta or implanted embryo. Some speed is important because the processes which allow the blastocyst to avoid getting kicked out by the female's immune system depend on this.
The human placenta, like other mammalian placentas, cloaks itself and the blastocyst/embryo from detection by the female immune system by secreting on its fetal side the same the secretion used for cloaking by the parasitic nematode worm. The cloaking is not completely successful, so certain immune attack T-cells of the female do attack the placenta and embryo.
The embryo chemically signals the fetal side of the placenta to produce the enzyme indoleamine 2, 3-dioxygenase, which catabolizes the local L-tryptophan (essential amino acid) in the female, which starves those immune attack T-cells of their essential nutrient, so that, to survive, they have to go into latency, a state in which they cannot function to protect the female or reproduce. In experiments with mice, when a chemical agent was injected into the placenta to stop the production of the enzyme, the immune attack T-cells were roused from latency and immediately attacked and expelled the placenta+embryo unit in all cases except those of embryos derived from identical twin inbreeding. The female's immune system is designed to preserve genetic integrity and to expel embryos that chromosomally deviate from it sufficiently in all but sex chromosomes.
So there are many points in between the semen deposit and implantation and the continuing implanted state of the embryo where things have to happen for pregnancy to occur. You would claim that, after the sex act initiated by one or both people, everything else is just "natural biological processes." But these processes involve distinct biological units. Sperm, ova, zygotes, morulae, and blastocysts can all live outside the female body and be preserved alive, if only by freezing. The sperm initiates touching the oocyte and is reponsible for its opening; the oocyte appears passive. The blastocyst implants; the female body resists rather than cooperating. The blastocyst makes the placenta, and it initiates use of some of the female's tissue, but the female's body does not. The placenta produces the enzyme that starves certain of the female's attack T-cells, but it is the embryo whose signal alone initiates this production.
If you would like to treat the zygote, morula, blastocyst, embryo, fetus as merely part of the female's body because they are inside her body boundaries, then the human female's choice to continue the pregnancy or end it is simply the first directly relevant human decision about control over her body after the decision to have sex. Notwithstanding the fact that a prior decision to use bc pills or a post-coital decision to use Plan B could prevent ovulation, a decision to use a spermicide could kill off the traveling sperm, a prior decision to have an IUD could increase the resistance of the endometrial wall to implantation, and a post-coital decision to take certain types of bc pills might also be able to increase that resistance.
But if you would like to treat it as a distinct human entity with a right to life equal to that of a human being, then it has the same responsibilities as other human beings not to abuse that right. It is then the human person initiating the implantation into the female human being's bodily tissue and suppression of her immune system. Those are things no other human persons have the right to do, even if they are mentally incompetent, hypnotized, or behaving unconsciously in sleep, and they are things that would justify the female human being's use of deadly force to stop regardless of their unintentional nature.
I'm standing by that.
