The father of the man I am currently dating is a reverted transsexual. When my boyfriend told me this, I didn't think much of it. I've had friends with transsexual siblings before, so it didn't phase me. After downloading some music on his computer, I opened the shared folder to find a bunch of porn files. Two or three were heterosexual, but the bulk, ten or more, were gay male videos
Before I try to answer your questions, I want to ask one of my own. What were you doing snooping on your boyfriend's computer and peeking at the kind of porn that he has there? This is really bad behavior on your part. You ought to apologize and take stock of your motivation. Even if people are in a relationship, they have a right to privacy and some freedom of expression. Unless he chooses to share the information with you, his sexual fantasies and porn collection are his own business. I'm sure there are aspects of your sexual preferences or erotic history that you'd rather keep to yourself. If these boundaries don't exist in your relationship, they need to be created and respected!
As far as we know, transsexuality is not an inherited condition. While it may have a genetic basis, no one has demonstrated a tendency for transsexuality to run in families the same way that alcoholism, for example, seems to do. The same thing goes for homosexuality. There's no evidence that a father who is transsexual would affect his or her children's gender identity. The sense that one is male or female comes from within. It's pretty much unshakeable by the age of two or three. And I believe it has more to do with the child's relationship with their own body than with outside forces like family dynamics.
The connection between fantasy and sexual preference or behavior is complex. Some people don't have sexual fantasies at all. Others will fantasize only about the things that they have done or would like to do. And there are other people who fantasize about erotic activities that they wouldn't or couldn't do in real life. We don't really know why your boyfriend downloaded the porn videos or what he was going to do with them (send them to a gay friend, perhaps?). But even if he has used them to make jacking off more fun, there's no guarantee that this means he is gay or bisexual. There are a lot of heterosexual women who enjoy gay male porn, and relatively few of them have gender identity issues or become male homosexuals.
You don't describe the other behaviors that are troubling you. I wonder if this is not the reason why you are conducting surveillance on your boyfriend. If your sex life with him isn't good, you may be trying to find out why. Have you tried simply asking him what these behaviors mean? Or telling him that you are dissatisfied or worried? I hope he will open up to you and ease your anxiety.
You have also asked what behaviors precede a major change in gender identity or sexual orientation. There are so many that it's really hard to generalize. Because the larger society stigmatizes same-sex behavior, coming out as bisexual or gay is usually stressful. It's not like you can go out and experiment freely, man-to-man or woman-to-woman, and then make up your mind. Just asking the question is usually enough to freak other people out and convince them that you are a raving homo. So people typically try to seek out more information, and perhaps they call a hotline or talk things over with a friend who can keep a secret. They may engage in phone sex or Internet flirting before they actually put their body on the line. A same-sex crush on someone who is unavailable is also common, and puts a lot of negative pressure on the individual's self-esteem. For men, it's fairly easy to find anonymous, casual sex with strangers