Basic Trope: No man wants to date a woman, no matter how attractive, who is stronger or better at fighting than he is.
Straight: Bob doesn't pursue Alice romantically because she outclasses him in fighting ability.
Alice is a 8-foot, musclebound warrior and Bob is a skinny, weak nerd that's barely the size of her arm who doesn't even look in her direction.
Alternatively, Alice is a white-belt in kung fu. Bob avoids and/or out-right rejects her because of the fact that she even knows kung fu in the first place.
Bob appreciates Alice, but her training gives her habits or personality quirks that he doesn't like, such as being smug or overprotective.
While Bob finds her superior fighting skills to be unattractive, it isn't necessarily a deal-breaker for him.
Bob knows that Alice is beautiful, but is intimidated by her strength and doesn't think he can pursue her.
Alice is a Blood Knight, a Sociopathic Soldier, or some other kind of extremely violent/overwhelming archetype which is extremely hard to approach to... Never mind try to engage in romantic interaction!
Alternatively, even if Alice is sweet, shy or even timid, her looks are intimidating enough that most men would be too scared of her to consider pursuing a romantic relationship.
She was just pretending to be helpless to get Bob's attention.
...and that practice includes actually learning kung fu.
But her fighting skills are one of the reasons, just not the only reason.
Alice only said she lied about her fighting skills to impress him because she didn't want him to turned off by said skills.
Parodied: Alice is beautiful, sweet, charming, and ladylike to a fault. However the minute Bob sees her use a Tai Chi move to fend off a mugger he treats her as if she has the plague.
Zig Zagged: Bob's attraction to Alice waxes and wanes according to the day of the week - one day he's turned on by her ability to fight, the next he's attracted to her physically but thinks she should act more feminine, then he likes her personality but is turned off by the muscles.....Ad infinitum.
Alice has just as much chance of winning Bob's affections as anyone else.
Alice tries to avoid guys finding out about her Action Girl credentals out of fear of invoking this trope.
Bob is initially intimidated by Alice's strength or fighting skills, but tries to be understanding and look past it.
Discussed: "So are you one of those wussy guys who's scared of a girl who can put you in a Full Nelson?"
Conversed: "Poor Alice, you'd think more guys would be clamouring after a woman that hot".
While it starts out as the standard "guy ignores girl who can fight" situation that's sometimes Played for Laughs, the story focuses on Bob for a while. When Bob sees Alice in a particularly messy fight, it triggers a horrific flashback: Bob lies in a bloody heap on the ground as authorities restrain a violently angry woman. It's revealed that she was Bob's abusive ex-girlfriend Susan, who was sent to prison for nearly killing him after an argument escalated into a physical fight. Bob's attitude and previous actions towards Alice are seen in an increasingly depressing light as time goes on.
Alice wants to pursue Bob romantically but is aware of his dislike towards her type. So she pretends to be weak and asks advice from her friends, in which Hilarity Ensues in their attempts. She successfully convinces Bob she is a weak female who needs to be protected and happily dates her. However, she begins to have identity issues because of her changes and breaks down after seeing Bob beaten-up (who feels morally responsible to protect her) by a gang and Alice is un-sure what to do.
Alice works to maintain Bob's trust after finding out the truth, and he eventually moves past his issues.