I agree with a previous poster that a lot of guys are into it for a dominant reason. That's why I like it. Guys are into it for a whole host of reasons, including even being dominated!
Another reason that I like traps is that I really like being an anal top, but I'm not attracted to men and very few girls in my past have been interested in it. Traps always are as is my current girlfriend, thankfully. Sure they have a dick, but for most guys, the thought of traps comes along once in a while, either fucking one or being fucked by one. Maybe it's just the exciting thought of doing something that's not "normal". Usually it's solved by a quick fap session and that's it.
Bottom line, I wouldn't be too worried about it. I know for me, whenever I think about traps, I just watch some trap porn, have a little fap and that's me for a few months until I think about it again.
Maybe if you guys talk it out sensibly and get a little more adventurous in bed, your fears will go away a little. Have an open mind, I'm sure you have your fetishes too that he might not immedietly understand. Explore each other! Leads to a happier (sex) life.