All blood is heavily tested regardless of whether or not you are gay or not.
new infections might not get detected in tests early on, which is why there are exemption periods. A few examples of exemptions in Sweden, in addition to those mentioned in my above post:
Recently gotten a tattoo: 6 months from the day you got the tattoo
Recently had sex with a
new partner (straight relationship): 4 months.
Had sex with a person of the same sex (men only): 6 months, this is any partner if I've understood the rules correctly, even a long time partner.
Had an open cut, where you've drawn blood (any injury where you cut yourself): up to 6 months
Spent time in close proximity (lived together with) a person with Hepatitis: 6 months iirc.
And so on. As mentioned by Madness, this might seem a bit over the top, but it is for the safety of the one receiving blood.
How do they know you're gay? Can't you just not mention it on their form?
Don't do this, be completely honest on the form, it's there for a reason. Remember, this is done for the people receiving the blood, not for people's own self esteem and getting pat on your back. It's not a right to donate blood.