1) Is this a valid reason to be thinking of an alternative?

I tell people, you are allowed to break up because they burn the coffee or don't wear matching socks. Yeah, it might seem stupid [in some cases] to break up over something small, but dating someone isn't a debt you pay when you die.

So, while I think your post comes off nasty and shallow, you are allowed to break up with her because she has confident strong features. Which reads "looks like a man and I don't want people to think I like men." But that's just me.

Personally I like a strong face, it has character.

2) What indicator should I use to know that this is a major issue?

I think it's a major issue on your part. Your [hopefully ex] girlfriend can't help her background, how she looks, or the fact your rude friend said she looked like a man. I think you need to consider this as evidence you didn't like her as much to begin with.

There was a guy who posted a week back [or so] about how his friends kept calling his girfriend mannish. He said he wanted to know how to defend her, not justify looking at other women. Your girlfriend doesn't owe you extras because she has a strong face.

That being said, it is a serious problem because you are looking at other women as possibilities, thinking of her as less because someone else said something. So I think you only valued her on shallow, comfortable ways and so should find someone who fits in with your lifestyle better.

3) Should I tell her about this thought?

No. That's just mean. You should only mention problems that can be fixed. She shouldn't have to deal with your negativity about her. Just break up and say it's because you have changed your feelings.