The reason people feel its weird is because its not currently common in society. Men, for the most part, don't worry about how they look (or are expected not to worry). Women in this society are expected to feel insecure about how they look. Its ingrained into them by adverts and a lot of thoughts about how women look are pushed onto them at an early age (e.g. "dont [act in a certain way] or no man will want you" type of things).
I'd rather no one felt the need to wear make up, then men wearing it as well as women. But there shouldn't be a problem with it at all. If a guy wants to make his eyelashes look longer and his skin look smoother, who cares? If you feel uncomfortable about it, how about considering that he's doing it so he feels better about himself and so he looks good to him. Your opinion is not an important one when it comes to people feeling happy with themselves.