But being catholic is not just 'believing whatever you want'. By definition it means having full communion with The Holy See. You can be a christian and twisted The Bible to rationalise your belief, but you cannot be gay and be proud of it whilst still being in full communion with The Roman Catholic Church.
It's possible that I am misunderstanding your point.
But according to canon law, once you are baptized you are Catholic - period. Baptism brings you into the Church, but the only way out is death or excommunication (which is so rare it almost doesn't exist.) There is no way to be "unbaptized".
So you can be a bad Catholic, or a lapsed Catholic, but you are still a Christian and as far as the Catholic Church is concerned. (CIC 845)
You may be confusing heaven with the Catholic Church. (Writing that makes me giggle.)
If you consider homosexuality a mortal sin, then you can argue that if someone dies in a state of sin that they won't go to heaven. BUT, according to Catholic theology, they will still be Catholic. In hell, but Catholic.
That is why I conclude that you can be gay and Catholic, regardless of your view on the rightness or wrongness of homosexuality.
Of course this is only relevant to Roman Catholics, and does not reflect my personal religious beliefs. I am just putting it out there since apparently I am just crazy enough to argue about religion on the internet.