Finding someone attractive, whether they're married/in a serious relationship/dating someone whatever... I would say is fairly normal. And it's okay because it's going to happen. But it just can't go beyond that. Sometimes it can be like a gay man saying a woman is beautiful or a straight woman saying another woman is attractive. Sometimes people can simply appreciate attractiveness and not take it anywhere beyond that.

But those who take it beyond that have issues they need to work on because some people ONLY go for someone who is otherwise taken.

You can find someone attractive/cute without even wanting anything to happen between the two of you.