The National Blood Service asks potential donors whether they have had sex (protected or unprotected) with another man. If a donor answers 'yes' to this, they are effectively banned from giving blood for life. This does not take into account their current lifestyle, and therefore the suitability of their blood.
Women who have sex with a a practising gay or bisexual man are only banned from giving blood for a period of 3 months. This 3 month period is roughly equivalent to 'waiting window' where the HIV virus is undetectable in the blood.
It is worth noting that all blood is screened for the presence of HIV, but because the waiting window is about 3 months, then this test cannot check for HIV infections that had occured in the 3 months before donation.

This policy is homophobic because:

• It enforces a blanket ban on gay and bisexual men regardless of present lifestyle (ie. you could not have had sex in the last year - way past the waiting window - but still be banned from giving blood)
• It allows high-risk heterosexuals to give blood but potentially bans low-risk homosexuals.
• It assumes that sexual identity, not sexual practices makes people high-risk, and perpetuates the myth that HIV/AIDS is a gay disease.

Some countries have lifted the ban on gay and bisexual men giving blood, and actually decreased the risk of getting HIV from blood transfusions.

The Donation not Discrimination campaign is ran by the National Union of Students (NUS) LGBT campaign.