I have a fairly extensive background in several Martial Arts and competed in MMA at an amateur level. I would be more than happy to give you some advice and info on the subject.
Regarding avoidance of masculinizing your physique: It depends on the martial art in question. Ones based on throwing another person's body (judo, juijitsu, etc) will have the side effect of building or maintaining upper body muscle mass. The ones that are closer in style to yoga (tai chi, aikido, etc) will have the opposite effect. Both can be supplemented or partially avoided with specific dietary changes, as certain foods have direct effects on hormone levels and muscle gain.
Regarding general fitness purposes: All of them, including boxing, and even taibo (cardio kickboxing) will give you a more than sufficient workout for your flexibility and cardio goals.
Regarding combat effectiveness in the real world, aka SELF DEFENSE, something we could all use a basic knowledge of for our personal safety against hate and other crimes. Here is where there are definitive pros and cons to each art form.
Kung Fu, Tai Chi, Capoeira, and their variants, while beautiful, graceful, and deadly at master levels of competence, are extremely hard to acquire even a basic level of competence against an assailant. A black belt or close to it is needed to provide an effective defense from an attacker.
Boxing, Kenpo, Karate and Taekwondo all provide at least an adequate defense improvement at even beginner levels, and are easy to pick up, but hard to master.
The supreme "street fighting" effective martial arts are those that incapacitate an opponent with minimal effort, and are effective regardless of size, speed, and strength of either you or your attacker, most involving joint locks. These are Mui Thai, Juijitsu, Brazilian Juijitsu, Hapkido, Aikido, and Krav Maga. If you noticed that the first four are quite popular in MMA, its for a reason. Aikido and Krav Maga use a lot of moves and principles that are banned in MMA, such as small joint (hand, wrist, etc) manipulation.
I have personally studied Karate, Capoeira, Aikido, Juijitsu, Taekwondo and Hapkido, but have only attained proficiency (Brown Belt level or higher) in the last 3. Krav Maga and Hapkido are both about as good of a one-stop shopping target as you can get. The others all have some area (grappling, striking, and joint manipulation) where they are lacking. Competition and International training in the military with other soldiers and operatives has led me to these conclusions. Just keep in mind in whatever you train in: There is always somebody better, so don't get cocky.
Feel free to reply or pm me for more info. Stay Safe.
HUGS!