Anyone can wind up in a situation where their spouse, significant other etc. leaves, cheats what ever. The best thing you can do is to make sure there are no issues that are unresolved between the two of you.
Its not as easy for trans people to find someone, so i think that when we do, we want to hang on to them and sacrifice some of what we want. If this guy is right, then he won't stray, but its better to find out how he feels before you are so deeply invested that a breakup tears you apart.
The only male gay couple I've known were partners for about nine years before they went they broke up. I'm not sure why.
Sam1234