the simple answer to your query is every religion (monotheistic ones, especially) forbids homosexuality.
so if a homo is gay and still calls him/herself muslim then he dont know what Islam teaches em.
ps. its the way it is. either follow it or either dont follow the religion and be what ever you want to be.
we have the same issue with ahmedia. they call themselves muslims but they dont believe on Khatam-e-Naboot (PBUH).
ps2. dave chappel put a very good analogy of this notion that closing your eyes doesnt make one who he/she is really not.