If you start having feelings towards another person outside of your own relationship remember that, three is a crowd! You need to be sure of your feelings towards your significant other before you start involving third parties and destroying your relationship.
It is normal to feel attracted towards somebody outside of your relationship but it should not transcend to more than a look. If you are truly in love with your partner you should feel as though nobody compares to him/her. That’s true love. Don’t act impulsively and remember that nobody deserves to be hurt.