I've not posted in this thread before, nor do I generally visit it regularly (I actually came into post somewhat about the bolded though so it's suitable that this is on the first page I visit), but regarding the bolded, I had always thought to myself that while I'm not 'openly out' (this was in part because there is still a niggling of the doubt that 'perhaps I'm bisexual and just haven't found the right woman yet'), that is to say that while I've commented to a few people about my sexuality it isn't 'well known' amongst those I'd talk to, I would mention that I am if the situation arose. Last Thursday though, during road running for a "Physical Education" class, while talking to somebody who I've been running with every class for the last year and a half, they mentioned a displeasure towards those of any alternative sexual orientation (of course he prefaced the comments about how speaking to somebody who is gay or bisexual is "nearly impossible because they're just so strange" [quite ironic I felt]' with "I've nothing against gays but..."). I had considered, after questioning him as to what he meant (for a brief moment that quickly faded) commenting that I was gay and he certainly didn't seem to have a problem talking with me; which is what I what I always expected myself to say if comments such as that were made in my presence. Despite feeling a sense of shock, tinged with a sense of betrayal (I know that's irrational though), and despite having seen him once again yesterday (for running once more) and the question as to whether I was an 'arse or a tits man' being aired, I still found myself unable to clarify my sexuality as I had always thought I would freely be able to do so; fearing that airing it, despite it always having been present (just unspoken), it would alter our interactions (not my sexuality, his awareness of it).
That's what I had initially came into the thread to comment on, mainly to ask whether other people would suggest that it's worthwhile to comment to him that I am gay to try and alleviate his prejudices or if it is not worth the risk of a ruined friendship. From a logical standpoint I'm quite sure the latter is the case (afterall, it was an entire year and a half until it was even raised in the first place so perhaps it can be ignored) but I was interested in others' perspective (I am only seventeen [until December] so I know ultimately it'll be a small squabble as opposed to any major conflict).
Of course, that is merely something anecdotal, unlikely to be what you were seeking when you were seeking an answer to the bolded question; so in more general terms it is generally, in my experience, the fear of rejection, harassment (physical or verbal), fear of being placed within a region's stereotype of homosexuality, fear of religous persecution, and the fear of rebuttals (which seems to generally be an issue with conservative families in my reading in which 'coming out' may result in issues such as being kicked out of the house without adequate time to search for any form of accomodation for example [my uncle on my father's side was disowned by my grandparents for example; I didn't learn he was gay, and married, until this year after having seen him only three or four times, on his own, in the last ten years]) seem to be the most common reasons, almost all that I've seen are about being fearful of the response due to the lack of conformity. I'm sure others more versed in the topic will be able to give you greater insight though.
EDIT: Well I've answered the second might as well go back and answer the first and third. Not OCD (Asperger's), and I'm a younger age than I assume you're looking for answers from, but I have had some struggles with regard to being able to accept my sexuality; the explicit material I watched was always homosexual in nature but even still I had, at times, convinced myself I enjoyed it merely as it was 'different' (despite knowing that it is not different when it's viewed exclusivel). This was somewhat compounded as the person I would consider my best friend is gay, and had made numerous comments with regard to his view of me prior to myself revealing anything, and I was wondering if that was perhaps influencing me in some way (it turns out it was influencing me, but only influencing me in that I'd an attraction to him). Regarding the third, it's something I've feared I may be doing at times (assuming you're referring to supressing an attraction to the opposite sex), but I think it's only a product of the slight doubt regarding my orientation as opposed to something legitimate. I'm interested in how others may answer though.