First thing's first: The ban isn't simply on 'gay' men, it's on any man who's had sexual contact with another man (MSM) in the last 12 months.

That means any man, whether they're single or in a committed relationship, must avoid all sexual contact with another man for 12 months in order to legally give blood. It also doesn't matter whether that man always uses a condom, is regularly tested for HIV and other STIs or has a note form his doctor. The same rule applies to women who have sexual contact with MSM. (ie a woman with a bisexual partner)

There are similar rules in place around the world. In the U.S., MSM face a lifetime ban. In Canada the deferral period is five years and in New Zealand it is one year. In some countries, like South Africa and South Korea, there is no deferral period.

The one-year deferral period exists for one reason, which on the face of it looks pretty simple: Around 80% of new HIV diagnoses come from MSM, and the people responsible for regulating blood can't risk HIV-tainted blood getting in to the supply.

But there's a lot more to it.