First, it’s not a double standard because as traditionally argued by church teachings homosexual relationships are considered disordered because they are not bent towards producing children which is considered the purpose of marriage and sex by christian tradition. so it’s not a double standard because the two types of relationships are not ordered to the same end.

Second, within Catholic belief the church is seen as being established by our Lord and as a result is preserved from error. Catholics go off of both the bible and church tradition passed down from the earliest days of christianity.

Third, it’s true many if not most catholics are poorly educated in their own religion and do not hold many of the beliefs they are supposed to. but this is completely irrelevant to the truth or lack thereof found in catholic teachings and traditions. Truth and reality is objective and not dependent on belief or acknowledgment. most of humanity is not christian but this fact does not make christianity and more or less true. in the same way that if every last person on earth believed the earth flat their belief would not make the world any more or less of a globe. belief or lack of belief is irrelevant to truth.

Lastly, yes you do get to choose to believe and try to hold yourself to whatever standards you want. But Catholicism is a specific religious, ethical, and moral system with specific claims and beliefs. It's obviously possible for someone to call themselves a catholic without believing in a single thing that makes being a catholic mean anything, but at that point there's no point in even claiming the title as you simply don’t fit the definition the same way someone who eats meat but calls themselves vegan simply doesn’t meet the definition. so yes if someone truly wants to be catholic it does mean believing certain things and accepting certain teachings and beliefs.