While certainly not agreeing with any one or idea that would assert that the entirety of an individual's sexual orientation is shaped solely by one instance of abuse, I also don't believe we can totally discount that individuals are affected. To what degree, I can only imagine, would differ from person to person.
I've come in contact, even dated several individuals who have been sexually abused. If only in the dating situations, this became apparent with time. Sadly, many of them were abused at a very young age. For this reason, I'm willing to contend that, if only depending on the age at which it takes place, it may in fact impact their developing sense of sexual identity.
Given that things tend, often, to get buried in our unconscious from an early age, only to reemerge later in life, sometimes in strange, often, if only initially, unrecognizable ways, it's no wonder that there are many who discover that who they thought they once were or preferred sexually, they no longer are or desire. And, so, they change. I think we have to especially allow for this change in cases where we come to discover some type of abuse in the course of that person's life, especially if it happened at an early age.
I recall hearing Oprah (who I'm not a big follower of) once commenting on her own sexual abuse and how in the course of it, when looking back on it, she recalled that it felt good. Obviously,this didn't change the fact that what happened to her and so many boys and girls is wrong.
Though, I do believe it speaks to the potential for individuals at an early age, when still developing on many levels, to make false connections as to who they may or may not be, if only in this case regarding one's sexual orientation. They then carry it into their more maturing years while, hopefully, becoming more aware of themselves. They, perhaps, begin experimenting and discover that what they once thought was, is not necessarily so and thereby begin to question, perhaps change, and in those rare cases they get "stuck".
I think it's when they get "stuck", due to numerous variables, is when potential problems can arise and is also reason for why many are not able to feel that they can be in a "stable" relationship or experience intimacy. If only due to intimacy requiring a considerable degree of transparency, how can one be transparent about something for which they are confused within themselves about? They're "stuck".
Anyway, I think it's definitely possible and does hinder individuals from living whole and happy lives.