If I own a bar, could I refuse gay people entry for being gay?

I live in London and often go out in places like Soho where there are numerous predominantly gay, or gay-friendly venues. I'm straight and have no problems at all with anyone with any label, or those without a clear one. I think life is too short and people far too interesting. However it has happened a number of times in my life that while trying to go into one bar or another, that I've been stopped and told I couldn't enter, and that this was a gay-only night. Is this, right? Is it legal? Should it be legal if it is? I can't understand why it should be wrong for bars to exclude gay people, women, black people but OK for gay people to exclude straight people.