The Constitution did not clearly prohibit what law? IIRC this was some sort of blanket anti-"gay rights" law that attempted to neutralize all existing and FUTURE laws relating to this rather wide topic. I think a pretty solid argument can be made that the Constitution prohibits laws that curb citizens' and their state legislators' right to pass new laws.
So you agreed with the liberals on Romer and disagreed with conservatives Scalia, Thomas and Rehnquist?
I respectfully suggest you read the opinion:
http://www.law.umkc.edu/faculty/projects/ftrials/conlaw/romer.html
and then come back and express your opinion. The link is to the Court's opinion and to Scalia's dissent. Romer was one of the most far reaching acts of left-wing judicial activism in the past 50 years.