Can you imagine what it would be like to wake up one morning to discover that it was suddenly illegal for you to be you? That the law said because you had blue eyes, or were short or because you wrote with your left hand you could go to jail.
If it's hard to picture, just take a look at this short video released today on the internet. There's a guy being beaten up who could so easily be you. Or maybe you could be the woman being subjected to all that verbal abuse.
The images had a really profound impact on me because for millions of people like me who just happen to be gay it's not some grotesque Kafkaesque horror story. It's the every day reality of their lives.
The character I'm best known for - Captain Jack in Torchwood - spends his life fighting every kind of threat to humanity you could think of. That's all fantasy, of course, but this is real. People are being killed and many, many more live in daily fear for their lives. And the threat comes from something that is much harder to fight than fictional aliens - homophobia based on prejudice, hatred and ignorance.
The video has been launched by an inspirational new global LGBT rights charity called the Kaleidoscope Trust. When they gave me the figures for just how many countries still make it a crime to be a gay I could scarcely believe it. No fewer than 78 states around the world criminalise homosexuality. In five of them the maximum penalty is death.
When we demand our rights to complete equality under the law, including the right to marry, we need to remember that for literally millions of gay men and women the right not be beaten up, arrested or even killed would be a massive step forward.
And don't think the situation worldwide is gradually getting better. I wish it were.
In Nigeria a new law that claims to be about the 'defence' of marriage not only bans gay unions, it makes any campaigning for LGBT rights punishable by five years in prison. In parts of the country, where Sharia law applies, the punishment for homosexuality is already death by stoning.
Uganda's notorious 'Kill the Gays' bill was amended under international pressure to remove the death penalty, but it will still make it a crime not to report homosexuals to the police. And for gay men and women themselves the maximum penalty is life imprisonment.
Just this week in Ukraine they are voting on a law to make all 'promotion' of homosexuality illegal. Local activists think it will be against the law even to say the word 'gay' in public. Ukraine was the first ex-Soviet republic to decriminalise homosexuality. Now they are planning to throw gay people in prison once again.
Millions of us will be in London this week celebrating 'World Pride'. There will be a lot of partying and a lot of fun, but Pride now carries a serious message too. It is a simple one. LGBT rights are human rights and human rights are LGBT rights. Hillary Clinton spelt that out in a brave and eloquent speech to the United Nations last year. Tomorrow she is being recognised by a special award from Pride London and the Kaleidoscope Trust. But today - American Independence Day - is an appropriate occasion to remember that freedom can't be taken for granted. It has to be fought for. And once won it has to be protected at all costs.
The freedom to express yourself and be who you are is a fundamental one. As long as there are people just like us who are suffering because their basic human rights are being denied we all have a duty to speak out.
I get up on stage night after night and can sing my heart out and nobody can stop me. But what if it was illegal for me to want to sing? Just think of something that is so fundamental to your nature that you couldn't change it even if you wanted to. Now imagine if that - whatever it is - turned you into a criminal.
I hope you'll take a look at the video and share it with your friends. And add your name to a campaign that has a simple message for all of us, gay, straight or whatever. It should never be illegal for you to be you.
To find out how you can save lives the lives of persecuted gay people across the world, visit www.kaleidoscopetrust.com
To see more about John Barrowman go to www.johnbarrowman.com
To be kept up to date on this issue, follow the Kaleidoscope Trust on Twitter at @kaleidoscope_T