I have to admit that I hate sport and don't see it as problematic their neither my son nor my daughter have any desire to participate it. DD has had it written in her school report for four years running that she is nervous of physical contact during sport. DS admits that he hides at the back during rugby and avoids getting the ball so that other people won't come and grab him.
Nobody in our house does any sport or watches any sport, including DH. It just seems incredibly boring. We will all exercise for fun, between us - riding bikes, swimming, walking, going up mountains, dance classes, snowboarding and horse riding. But none of us would ever compete in sport or take part in an organised event. So I don't see that women or girls not participating in sport is problematic; I see it as men over participating in an activity that I cannot see the appeal of at all. Is sport held up as being a great thing because it is associated with men, or because it has some kind of universal benefit that I'm not seeing and women are missing out on?
I wouldn't claim that it is more natural for be women to be less muscular. Clearly it depends what you do with your body. But women's do store fat more in particular areas and testosterone differences make it harder work for women to become muscular than it is for men. Women's bodies are different to men's and do require a higher proportion of fat to remain healthy. But there is a great amount of room in the middle between having too much fat and having too little fat for women to have a variety of body shapes that are healthy, including those that are more muscular than women in our society usually have. But you have to work harder than a man would have to to build up that muscle tone. And so while muscular female bodies should be seen as perfectly acceptable, I don't think something that requires a lot of effort to modify the body should be held up as being a new ideal.
And while many women may want to define themselves by other things which is fine for them, I do define myself primarily as a woman who has been pregnant, breastfed and is a mother. But I do think that there is an argument to be made that it is feminist to do something that is just about you as a person, is about you enjoying yourself, has no benefit to anyone else and isn't a form of paid or unpaid work. Women don't spend enough time doing things purely for their own satisfaction and enjoyment. And weight training would fall into that category.