Its very simple.
Subjective morality says that morality is a matter of an opinion.
Thus, only two possibilities can follow:
1. Some moral opinions are objectively better proven than others, thus they are objective ones.
Or
2. All moral opinions are equally unproven.
To avoid conceding to an objective morality, some naive person could say "all moral opinions are equally unproven".
But that results in a very simple objective conclusion:
1. If all moral opinions are equally unproven, then all moral opinions are equal. This is an objective fact.
2. If all moral opinions are equal, they all deserve to be equally applied.
3. The only way all moral opinions can be equally applied is if all have same amount of area for application.
4. Thus, each person can own his body and apply his moral opinions to it.
To go against 4 would yield an objective contradiction.
If one person was to attack other person, his moral opinion would extend beyond its borders and claim that it deserves more area than other moral opinions, which it, by objective fact of being equally unproven and thus forced to be equal to others, cannot have.
By being equally unproven, no opinion can be above other, thus cannot logically get more than other in terms of distribution of area it rules in, but it can only have same.