IMHO... No it's not. If you love your SO and if you hope she has as much respect for your feelings as you SHOULD have for her's then the answer to your question should be clear too you. if you have some overwhelming urge to cheat on your relationship, then leave her. In the long run it'll be easier for both of you if you leave.
In another thread here on TERB, a guy who admittedly "hobbies" on his wife whined when he found out that his wife was cheating on him as well and decided to leave him. I hate to be crass but it serves him right. She probably had an idea what he was up too and decided to give it a try herself and found someone who she wanted to be with more than her husband.