In a contractual sense, the individual the FEMALE made the compact with was not the blastocyst, the fertilized egg or even the sperm, but rather the MALE sexual partner. If both the MALE and the FEMALE had agreed that the FEMALE would become pregnant and carry the resulting unborn human organism to term, then the contractual basis of the argument could be considered to be valid. In general, that's not the case in the circumstances of most abortions. An abortion requires the consent of at least the female party that made the decision to engage in sex (and in general, both parties are in agreement).
Block's evictionism argument is at best a simplistic and emotional metaphor. It falls apart in the sense that the associate the invited MALE brought to the FEMALE's home is not simply refusing to leave the house - but has tapped into the home owner's blood supply, is extracting nutrition from her body and is dumping wastes into it. I've heard that the progressive's are working on a biological evolution that acts in a similar fashion and will allow welfare recipients to tap into the productive members of society and feed on them and dump wastes into them in the same fashion (another simplistic metaphor that really only partially applies, but conveys an emotional rather than logical message).
There can be no contract between the FEMALE and the unborn human organism within her; that organism is simply incapable of making such a contract. It wasn't invited to implant itself in the FEMALE's womb, it simply did so without consideration to consent (there was no negotiation between the Blastocyst and the FEMALE). There might be a case to be made that the FEMALE's acquiescence to the fetus being implanted in her womb for a given period of time constitutes consent to it being there - but it's the same type of logic used by squatters attempting to seize the property of someone else.
Without a consensual agreement, I fail to see where the FEMALE is under any obligation to continue feeding the unborn human organism (UHO - an acronym that saves me typing) or accepting waste products from the UHO. Given that, what method of separating the two would be acceptable? Would simply severing the umbilical cord and leaving the UHO intact be sufficient (or would that be an act of aggression - seeing how umbilical cells are of the UHO's DNA)? Could we sever the placenta at the decidual layer (where some of the cells are maternal cells and others are those of the UHO). Or do we have to resort to surgically severing cells which only belong to the FEMALE? In any event, you eventually get to some act of separation that is not an act of aggression to the UHO (but which poses greater and greater risks to the FEMALE).
I can't make a case that the FEMALE can be construed to be under any obligation to go through the birth process to expel the detached UHO (birth being a riskier endeavor for her than abortion - and her medical choices are hers to make in any event). The scenario becomes one of a UHO detached from its nutrition source but still in the womb, and dumping its wastes into that womb (and isn't that an act of aggression by the "innocent" UHO against the FEMALE). Does the FEMALE have to wait for infection and sepsis to begin before defensive action can be taken against the source of the infection?
Basically, we're not talking about an associate of an invited guest who's refusing to leave your home. It's much more complicated than that - the metaphor fails.
I tend to agree with Wendy McElroy's reasoning on abortion (
here and
here). It's emotionally troubling, particularly in the case of later term abortions; but appeals to emotion should not over-shadow self proprietorship. Ultimately what a typical pregnancy is about is an autonomous individual continuously consenting to host a non-autonomous individual until it becomes autonomous - no contract involved. Once autonomy of the UHO is achieved, responsibility for the now-born (but still dependent) human organism can be transferred to others if need be. But until that point, biological reality leaves the FEMALE host as the only option - and its the choice of that FEMALE Host as to whether she wants to consent to the next microsecond of hosting the non-autonomous UHO.