Consensual sex between two unmarried adults violates no one else's rights, unlike bigamy and adultery, both of which violate the very public institution of marriage.
Well,how do you feel about polygamy or incest then? You think a divorced fellow should be allowed to marry his daughter? The age of consent is 14 in Hawaii.
And I do see a reason for these laws apart from imposing subjective morality on someone who may not agree with it.
If you have a couple of aging homosexuals living together and leaving everybody else alone, I really don't think anybody -- including the law -- should bother them. On the other hand, if they are having a orgy every other weekend featuring anonymous sex they may just be creating a wee bit of a health hazard -- especially if some of the participants go home to their unsuspecting wives and girlfriends. So much for the "gay gene."
And I think a law might be a tool for dissauding an authority figure -- priest, college professor, employer -- from taking home an 18-year-old with a poor self-image and with help from drink and drugs convincing him he really doesn't like girls.
Regardless, the pros and cons should be considered by an elected, changeable legislature, not by unelected, appointed-for-life judges.