Of course its natural to be attracted to someone when you are in a relationship, and of course the initial 'fizz' of talking to someone new and attractive might be more of a hormonal rush than the comfy 'whats for dinner' with your partner.
HOWEVER its not acceptable to pursue that frisson whilst in a relationship. The relationship has a depth and intimacy that is built from long term knowledge, trust and understanding. TRUST being the big one. Trust and love, which goes far deeper than a passing frivoulous thrill. You dont get to have both. You dont get to have both by keeping it secret, and trying to justify it to yourself. Keeping it secret and doing it anyway is dishonest and breaks the trust.
Be honest with yourself. If you yearn for the frisson either try to find it again with your partner or leave her and do it openly. It will come and bite you in the arse if you dont. You may have got away with it this time, but you have broken your relationship a bit, whether she beleives you or not. If you have been totally open about your role in what happened then she is going to always have a little doubt now, is always going to trust you a little bit less. It might eat her up and build into the end anyway. If you have lied then you know that you have lied and the relationship is now a little bit of a sham, you have lost a bit of intimacy, because at the back of your mind you know you are not honest with her. That might eat you up. It might make you think you got away with it and can do it again. Its changed, and thats your fault.
The object of your desires may not take it lying down. she may be humiliated and want to seek revenge. she may contact your partner, she might bad mouth you - she may tell everyone who listens how you told her you were attracted to her and invited her on a date.
laa de da.
not such a simple little harmless thing after all, was it.