I believe there are many people who have a list of characteristics for their "ideal mate". However most people end up marrying those who lack many of those traits. Some call this "settling" while others call it becoming "realistic" and still others had hoped that with time they could get their mate to "change" or take on those desired traits.
It's not uncommon to feel attracted to someone that displays those traits that your spouse or significant other lacks. The deciding factor of whether an affair of the mind/heart or body ensues depends on how important those traits really are to us.
Another major factor is our perception of the current state of our marriage, our relationship or our life in general. If we are not feeling good about things on the home front it would take very little for us to see another person as "a breath of fresh air".
Sure signs that you are attracted to someone are you think about them often, you fantasize about them, you find yourself dressing to impress them or elicit compliments from them. If this is someone at your job and suddenly you go from saying; "Thank God it's Friday!" to saying: "Thank God it's Monday!" because you can't wait to see them again; it is a sure sign you are attracted to them.
It's completely normal to be attracted to people other than our spouse. Just becomes one gets married does not mean they go blind or deaf. You buy a new home or car and yet you still can admire the beauty of other homes and cars. However we must avoid doing things that would jeopardize relationships we value and took years to build. Most affairs begin with having "secret friends", saying or doing things with someone we'd never would if our mate were sitting right next to us. We kid ourselves by saying "As long as (this) does not happen then it is innocent." The word (this) is different for everyone. People draw their own lines; flirting, secret lunches together, emails/texts, phone calls, emotional investment, holding hands, kissing, or having sex.