Maybe I have my manly moments lol but Ill explain to you why I check other people out even while in a relationship, Im a little blunt though.
I will see a guy/girl bent over their car for example and think they have a nice derrier, may not say it allowed but for two secs it looked good. Like some of the girls said, its like appreciating what your looking at, without looking at it from being in a relationship with that person.
I do what your husband does and did it while I was in a relationship, I may have been attracted but you always remember the person you're with. Flirting for example is healthy, never lead any on and there was never any contact but it spices things up abit.
Not only guys eyes stray, I know alot of woman who do it as well.
I wouldn't worry, its normal, its like you are subconsciouly single but consciouly taken, our bodies,"some of us: are almost programmed to do it.
It doesn't mean you are fat or ugly, just remember he comes back to you at the end of the night.