Yeah, I think you are worrying too much.
For starters, I don't think the whole gay, straight, bisexual orientation is always as easy as one of those three options, and the lines tend to get blurred a lot when things like transgender / transsexuals are involved. Still, I can tell you a few things that might ease your mind.
First, if he hasn't cheated with another girl, and if he has given you any reason to suspect he's cheated, or ever will, this whole shemale thing should not be much of an issue. Sure, transgenders and transsexuals are harder to come by than regular girls, but come on, that doesn't mean that's the factor that will push him over and cause him to cheat. So you don't need to worry about that.
I also want to continue talking about the interest of shemale pornography, I once watched some incredibly long video where some eggheads were talking about what they've studied in people interested in shemales. Apparently, the majority of them identify themselves as straight. They went into the reasons why, and part of it has to do with visual queues, optical illusions and other various psychological trickery, but eventually they just get into it. Apparently a lot of them are mostly just into watching it.
As for the the strap-on. Don't worry about that too much either. I mean you don't gotta do it if you really are that opposed to it, but really, it's not as uncommon as you might think. There are a bunch of couples out there into pegging. It's supposed to feel really good for a guy. Guys have this sort of G-Spot of sorts in their asses, that's hit through anal stimulation. It's in the prostate glands I believe. Anyway, I've been told that if worked right, you can actually give a guy an orgasm, and even make him cum.
All in all, there are studies by smart people saying that this kind of stuff is apparently not all that far off from normal, so really it's probably not something you should worry too much about. As I said, if he hasn't given you reason to suspect cheating you shouldn't worry about it too much, now.
I think you really should talk to him, though. I think you should talk to him about ALL of this, and if he gets embarrassed about the shemale porn, try to calm him by making him feel he has nothing to be ashamed of. Then talk to him about ALL of this, but whatever you do, try not to launch a full offensive. You don't want to sound accusatory. Come from a stance of caring. A place of concern. Just let him know you're insecure, and you're really just trying to understand, or get some reassurances.
If you can't talk about this, then you have a problem. You NEED to talk about this.
Best of luck to you.