Anyone who has studied sexuality for any period of time will tell you that a person's sexual preference is not a binary situation (gay/straight) and that people have a varying level of attraction to both sexes; and there are cases of individuals who are highly attacted to both sexes, and other cases where people are not particularly attracted to either sex.
Now, being highly attracted to the opposite sex and having a low level of attraction to your own sex is the dominant orientation for the world and represents the vast majority of people. At the same time, being highly attracted to you're own sex and having a low level of attraction to the opposite sex represents the smallest minority of people. This middle ground of people who have at least a moderat level of sexual attraction to both their sex and the opposite sex are the ones who end up being "Converted" or are in the high profile cases of people who had long term relationships (gay or straight) and ended up "switching" later in life.
On top of this, there is evidence to support the notion that your sexuality is not "Set in stone" and it changes throughout your life; the kinds of women/men the typical 14 year old is attracted to will change dramatically throughout their life, and the level of attraction toward their sex (or the opposite sex) can also change (just ask men who have been in prision).
This isn't to say that coversion will work on many/most gay men, or that it is a worthwhile effort to try, but it is not as silly and unfounded as some people would want you to believe.
