Have you heard of different cultures? Not everyone is from a British culture.
"Cultures" are completely irrelevant to this. We all have "cultures". If you are using that term as a euphemism for people coming from different countries then I have family and friends from all over the world. If you are trying to use "culture" as a euphemism for religion-based homophobia then as someone who is gay and wants to marry a woman you're going to need to break free from this and stop pretending it's acceptable, because as you know, it isn't.
The facts remain:
You are living with parents who have no idea you are gay because you like that it gives you a more comfortable lifestyle.
You claim you want to marry a woman but won't even admit to your family she exists so clearly you are not mature enough to enter into a marriage.
You say you have sufficient money to leave and be honest bit you choose to continue to lie to your parents and behave in this dysfunctional way to your supposed-future wife.
You don't appear to have ever lived independently.
You've made comments about marriage that show you have no understanding of what it is, as a legal contract, and what that entails in terms of assets.
You've made immature assertions about you being unable to behave in an honest manner and treat both your family and partner with respect by being honest because of some clinging to a pointless belief that you must marry someone you've never spent a significant amount of time with in normal day to day living situations, on the basis that your "culture" says you should, even though this would obviously be very unwise and is pointless to do so on that basis when marrying/ living with that person is a far bigger breach of your "cultural" issues anyway because the homophobic family will take far greater exception to her being female than to whether you cohabited before marriage, as you know. And you don't even have to cohabit, you could just move out (which you can afford) and try living as an independent adult and tell your parents the truth and live with some integrity, then date your girlfriend openly and stop living a double life.
There are no "cultural" reasons to justify this. It is the antithesis of what "culture" means.