[Eternal]

It's normal to find other people attractive, but I think something you should think about is how you'd define a crush. To me, having a crush means you have feelings for someone. If you just think this guy is attractive then I don't think you have anything to worry about. Being a relationship, especially a long-term one typically means you don't have eyes for anyone else. But even if you're madly in love it doesn't suddenly turn you asexual for everyone else. You'll still find people attractive and you'll still see their good qualities.
If you think you may be developing feelings for someone else, then I recommend getting some space for them if things are going well in your current relationship. As you said, it's not worth it to throw away this relationship that's working than to risk getting into one with someone you don't know.