The ONLY reason you and many believe this to be true, is simply for the fact that you have already seen her with makeup on... which is her UPGRADED version of herself.
would you switch your phone or computer for the first edition of its kind? Why don't you have a flip phone instead of a smart phone? ... for the obvious reason that you have been exposed to the better of its kind. Men have remained the same since forever!! yet girls have been altered since the beginning of time, because "society" decide that they should have this and should be added with this and this... so now you have perfected women with alterations, which now you can compare the natural with the upgraded...
You just cannot compare women to men this way... If girls would have NEVER been upgraded, you would find any girl just as attractive as you do now even without all these superficial shit... but since you've seen it, your eyes are appealed by it and when seen without it your eyes are not longer amazed quite the same... It's common sense... but people (mostly men) will never see or realize this until pointed out. ... Even the most beautiful woman in the world without makeup, if added just the tiny amount needed will be even MORE beautiful than her natural self... so now you can compare that to her natural self and come to the same conclusion that she looks less attractive without makeup... well duh! that applies to anything that has been upgraded.
In my opinion both genders can look good but since I'm straight I prefer looking at a woman's face. Makeup tends to give the user smoother skin but I wouldn't say women look less attractive without makeup. Both genders look good without makeup. It comes down to preference, when determining who is more attractive.
Make up just makes the women feel more confident and pretty. It's just like if men were to grow their beard and they liked that look, it makes them feel grown up, mature and handsome. If they didn't have the beard they would still be themselves, if women didn't have make up they would still act like themselves and I don't think it affects the way they look that much.
I think it depends on their age, really. Young women look just as gorgeous without makeup. When you get a bit older you need a little makeup to keep up with your manfolk who can so easily hide behind a nice beard (or societal acceptance of manly ruggedness...) I've lost count of the number of times I've caught a brief glimpse of someone and not known if it was a young man or a middle-aged woman... I guess we are trained to expect a woman's skin to be much smoother
Neither women nor men are more attractive in general. There are equal amounts of attractive and unattractive people from both genders. Plus, makeup doesn't magically change someones face, it just helps enhance what's already there.