By my read, the Constitution does not forbid states from passing laws regarding sexual behavior. This is obvious to me, since there were dozens upon dozens of such laws on the books during the framing of the Constitution and in the decades that followed.
Should there be such laws? On that, I am torn. I am not convinced of the utility of the laws. I see a benefit in a society setting standards of acceptable behavior, but I also see a detriment in that when laws are enacted that are difficult to enforce and likely to be ignored you foster a culture of disrespect for the law. I am convinced that not every gay person is born that way; I am not convinced that none are. I know that I consider it to be sinful behavior, but everyone sins, and not every sin should be a crime. My ambivalence on the matter of laws of this stripe lead me to appreciate federalism. If California wants to have it be legal, but Utah doesn't, I am comfortable with it.
I do know that I think that incest SHOULD be illegal, as should bigamy, polygamy, and that adultery should have legal consequences in matters of divorce and separation, so I am very uncomfortable with the idea that the legal basis for such laws being struck down.