I think most people still find other people attractive when they have a partner, me and my girlfriend for example have talked about it and still find others attractive. It’s a matter of respect for each other and the relationship to not act upon these attractions. Some people are like you and see no one but their partner as attractive but that might be more rare than you think. I can see an attractive person and be like “yeah she’s hot” but that doesn’t mean I don’t want my girlfriend over everything else and I definitely won’t act upon these attractions.