Some of it is baseline socialization. Depending on the type of socializing the gent often takes, this may have an impact on his patterns and styles of speech. However, I would go so far as to suggest that some of this is just an inborn trait. I've spent a lot of time around kids as my best friend has taught many ages of children, and I helped her with running a daycare many years ago when she was in college. I was literally blown away at how obvious some kinds, aged 6 even, were as far as their sexuality. I'm not suggesting that children are sexually active, but being gay myself I recall very clearly that at a young age I was different -- and now I wonder if I was as "gay acting" as some of these boys and girls were. It can be very apparent.
This goes the same for men and women I have known from around the globe, certain traits are shared -- even physical traits. The term "gay face" (which is stupid but cracks me up nonetheless) to describe men and women with "stereotypically gay features" really isn't that big of a misnomer, it's true in some cases.
Feminine speech patterns may just be inherent to some gay men, the way masculine speech patterns are inherent to some gay women. It has nothing to do with their personal identification as their gender or sex, or their sexual orientation, it's just something that occurred in development. Some people are bothered by it, because they don't think men should be like women (because femininity is frowned upon, or otherwise), and it seems to be less of an issue with lesbians -- they don't care if their woman speaks like a dude!
There are many reasons, but what is most important is to remember that discriminating just for the sake of discriminating, especially within the gay community, is useless and ultimately tasteless. I understand that we all have our preferences, and things that turn us on and turn us off, and that some things are attractive or they are not, so I'm no advocating that your preferences are ill. I'm just suggesting that the discussion should continue and progress to the point that it's not a bad thing, it's just a different thing...