The Kansas Supreme Court Friday unanimously struck down a state law that punished underage sex more severely if it involved homosexual acts, saying moral disapproval of such conduct is not enough to justify the different treatment.  In a case closely watched by national groups on all sides of the gay rights debate, the high court said the law suggests animus toward teenagers who engage in homosexual sex.  The case involved an 18 year old man found guilty in 2000 of performing a sex act on a 14 year old boy and sentenced to 17 years.  Had one of them been a girl, the maximum sentence would have been 15 months.