I don't want to say it's okay, because "okay" implies that you can just let it continue. It is normal, though.

People don't stop being attracted to other people just because they are in a relationship. We're still human, and a lot of people in the world will have qualities that draw us to them. I love my wife, for example, but I still turn to jelly whenever I see Amy Winehouse or Rhea Perlman on the TV. You can't really turn that off. The trick is what you do about it. Or don't do about it.

How do you handle it? You ignore it. You accept it as normal but you don't indulge it. You put it aside and focus your attention back on your committed relationship. If you have trouble ignoring it, then avoid being around the object of your crush until it passes, as most crushes will.

If the crush doesn't pass, if it persists for months and you can't stop thinking about the other person, then you've gone beyond a crush and you might have to consider what your heart is trying to tell you. Many a person has met his soulmate (if you believe in that sort of thing) while committed to someone else, and I don't want to tell anyone to ignore her heart.

But be very careful; the grass is always greener, and just as many people have given up a great love for something they thought would be better but wasn't.