I think it is natural to notice other physically attractive people (unless one is pre pubescent, blind, or dead!). Men in particular are very visual, and in modern culture, it seems to be a rarity to find women who are coy, aloof or nonchalant about such things.
My husband is not obvious if he finds another attractive and in almost 18 years together, I think there have been a handful of times he has made any comments indicating he finds another pretty. I on the other hand don't find the need to hide and am pretty outspoken. I don't often make comments - someones looks and physique really have to make me go "WOW." My husband does not seem to mind.
Whether or not it's disrespectful I think depends on a couple things....one, being how the other persons beauty is admired. If a man (or woman) admires another they find attractive in a way which suggests they are "lusting" after that person, then of course it's disrespectful. If both people are confident (in contrast to insecure) and one of the two who make up the couple, points out they could not help but notice an attractive person, it shouldn't be a big deal. Yet if someone has insecurity issues to overcome, then it would be wise to refrain from making comments or even the temptation to be obvious in noticing anothers beauty since to do otherwise would be disrespectful.
Some do attempt to be objective about such things and even if we are objective and conclude although we may be ok with it, we have to understand others may not be. Respect does include taking anothers feelings or reactions into consideration.