A homosexual person can attend Mosques just like anyone else, given they give due respect to it as a place of worship, as is expected of any other heterosexual person.
(yes, non-Muslims can not only visit Mosques but worship too; something proven from the practice of Prophet Muhammad, Peace Be Upon Him).
No matter what the sexual tendencies of an individual, Islam teaches that we must respect everyone. This does not mean that Islam condones their beliefs about sexuality or their sexual lifestyle. Similarly, Islam does not permit oppression or punishment towards anyone, including homosexuals, but this does not mean it supports the acceptance of homosexuality as a norm.