Your goal would require years of dedication, high level training, and a healthy nutrition plan.
Do you have a plan?
You’ve mentioned wanting to feel like you can protect yourself. Have you considered martial arts training? Krav Maga is popular amongst women. You will get in shape as a side product of learning some practical skills.
Actually, my goal is not realistically attainable, but that always gives me something to strive for.
That is a great idea, self defense training.
And I don’t care what guys think, I can’t live my life according to others opinions.
And I don’t care what guys think, I can’t live my life according to others opinions.
And…
That’s your answer right there.
That’s a relatively new thing, historically speaking. Even within my lifetime, Linda Hamilton in T2 went from being a bit freakish to being “in good shape”. And beauty standards have loosened up a LOT in the other direction as well- think about the absurd news/gossip about how Kate Winslet was “fat” in Titanic. Nobody would bat an eye today.
I think that a lot of guys have a very narrow idea of what is and isn’t feminine, and having visible muscles falls outside of that, and firmly into masculine territory.
As has been said, it isn’t really possible to get more buff than one intends to, if such a threshold even exists.
I think it’s less new than more common. As a comic/sci-fi/fantasy nerd since the 70s, I noticed a lot of my cohort liked buff women. Probably in no small part to our constant exposure to scantily-clad heroines. It always struck me as odd since most guys I knew were both out of shape and vehemently against working out; being fit made me an oddball among the oddballs.
I don’t think any concern is truly warranted, given the OP’s apparent age judging by post 13. We’ve got a thread about middle-aged men resorting to steroids to gain muscle; I don’t think a post-menopausal woman on estrogen supplements will be getting jacked any time soon.
A lot of fantasy art from the late 70s through the 90s seem to have athletic women (and men). I was playing a pinball table from 1979 called Future Spa that features… a lot of that artwork. 
Nowadays there seems to be a nerd subculture that reveres muscular women and want to be bench pressed by them but they’re totally straight they swear. (no need for bis like me to keep that pretense)
?? What is “un-straight” about a (male, I presume?) nerd wanting to be bench pressed by a muscular woman?
Nothing wrong with it whether it’s straight or bi or gay, natch, but ISTM that male sexual submissiveness to a physically dominant partner (if that’s what we’re talking about here) is not automatically code for male homosexual inclinations, which is what you seem to be implying. Some straight guys just like powerful women.
“Nowadays?” That’s been a part of nerd culture forever.
You go girl! Get strong. And do consider martial arts if you want to defend yourself.
This bears repeating. If the BF says “don’t get too buff”, promise that if you manage to get a little buff, you will revisit the issue. Getting buff enough to be seen as too muscular is really hard work, I remember the days of thinking “I don’t want to get too bulked up” as if it would happen by accident. It won’t.
Work out as hard as you want for as long as you want, you’ll be fine.
Very late reply - they are even rarer in places where there are no wolves.
I do think wolf re-introduction is a good thing, but that is much easier to say from my suburban home (although we are seeing coyotes and foxes enough to concern folks with smaller pets).
Wolves prey on coyotes and foxes 
Very late to the thread …
I am to some degree surprised by how the pernicious strength training will make women “too buff” myth has such endurance.
As to the OP, yeah you may need a better class of boyfriends, dunno, but their insecurities are not your problem.
To the myth, beyond what has already been stated:
There are many reasons to strength train and different approaches to best aim for different goals. Hypertrophy training, trying to get as visibly muscular and large as possible, is one goal, with bodybuilding shape as a subset. One can also train for strength maximization, power maximization, or muscular endurance, with different approaches. Or just on the basis that strength training once or twice a week increases healthspan and life expectancy, preventing sarcopenia and frailty. Or that it is FUN! Strength and size/definition do not necessarily go lockstep together.