At least in Europe, you're not allowed to donate blood/bone marrow if you've recently had unprotected sex. I think moonogamous gay men are allowed to donate, but not single gay men.
Nope, it's not related to monogamy at all; if you're a man who's had sex with a man (protected or otherwise) in the last year, you're ineligible. Note that this is also regardless of stated sexual identity; the category is actually 'men who have sex with men', not 'gay men'. http://en.wikipedia.org/wiki/Men_who_have_sex_with_men
In the US, the Red Cross requirements [0] prevent you from donating if you've had male-to-male sexual contact in the last 34 years (since 1977). There are several other "since 1977" criteria, including being paid for sex or living in certain parts of Africa.
