I think it's less about strength and more about martial skills. In a contest of pure strength, women will always lose. However, in terms of martial skill, a woman skilled in certain martial arts has a better chance of defeating an unskilled opponent.

However, that's only if they practice "soft" martial arts that are designed to use an opponent's strength against them, i.e., Judo, Aikido, Jiu-jitsu, Tai Chi, etc. Martial arts that require muscle power to work, such as Karate, Taekwondo, or Muay Thai, will be useless for a woman fighting a male opponent regardless of skill.