Paranoid about the deferral period on blood donation..

So I've been thinking a lot lately about the law in the UK (or only GB?) and in the USA where men who have sex with men have to wait 1 year before being able to donate blood.

This thing has been causing me sleepless nights and pretty much destroying my mental health. I've been told countless times that it's a discriminatory thing, but it still bugs me that, given that these two countries are somewhat seen as the 'leaders', they still haven't lifted the deferral period before donating blood.

I live in Italy where homosexual couples still don't have the rights to marry, but MSM can donate blood. Even Russia has allowed MSM to donate.

Now I'm wondering, why do some countries have gay marriage legalized but not blood donation for men who have sex with men, or they allow MSM to donate but not homosexual couples to get married?

Do you think there's a reason behind the deferral period? Or is it just a discriminatory thing that still unfortunately exists?