Here's the scientific reason:-
It is perfectly alright to watch girls as long as they are not feeling uncomfortable with it. Just a presence of attractive ladies around boosts up the mood as male's body releases certain good hormones that set the mood and prevents depression. Psychologists found out that just 5 minutes talk with an attractive girl raised the testosterone levels by 14% and anti stress hormones by 48%. Men would admit that the presence of just one attractive girl in the office improves their mood and overall productivity too.