[So, what's wrong with consentuality aside from leading to possibilities you or I personally may not agree to?]
Plenty of things. We all understand this concept in less hot-button issues.
Every state has statutory rape laws that set a lower limit below which a person cannot give consent. If an adult has sex with a minor, it is rape. Even if the kid tells you he or she wants to do it. Most people also understand that it is just good policy to forbid the president of the firm from having casual sex with the interns.
Why is that? Because the power differential is so great that the whole concept of "consent" gets skewed.
It is also illegal to engage is sexual practices that endanger the life of one of the participants, even if the one put in danger consents. If someone gets rushed to the emergency room with a dildo lodged in his throat and a knife stuck up his ass, telling the cops that "he wanted me to do that" isn't going to get you off.
It is clear that consent, in and of itself, is not the beginning and end of acceptable sexual behavior. Other factors also must be taken into account.
So with gay marriage, polygamy, or what have you. A person can argue that these practices should be tolerated by society, and maybe they should. But just because all the participants consent to them does not, in and of itself, demonstrate that it is the case.