Homosexual Jews should certainly keep mitzvot, no question about that.
Officially sanctioning a lifestyle choice which the Torah condemns in the strongest terms is altogether different.
The synagogue doors should be open to all people, including Jews who are gay, intermarried, sabbath-desecrating, etc. How exactly to make these people feel welcome and how to be inclusive without appearing to sanction their choices is a different issue, and one that requires much thought and sensitivity to a wide array of halakhic factors, including ahavat yisrael, kavod habriyot, dignity concerns, as well as marit ayin concerns.