I know that nouns that end in certain letters become by default male, but if a noun does not end in any of these letters (e.g., Бостон) what gender does it take by default?
Try be thinking in Russian.
We don't place a (grammatical) gender in a person or a thing. The question that would be surprising to people, is what are supposed to be masculineness or feminineness associated with seasons
зима, осень? Nothing.
She lived as a woman but presented us by her title as being a man:
... она дала такой ответ: «Я вас оставлю директором, а сама буду генеральным». [М.Андреев. Золотая рыбка областного образования, «Воронежские вести», 2003]
She told about herself (or men told about she) expressed it by masculine forms of words and this is norm:
Будучи его учителем, тетка была и товарищем его игр. [М.Горький. Фома Гордеев (1899)] ... свободное обращение детей с учителем, которым в тот день была Прозорова. [И.В.Омулевский. Шаг за шагом (1870)] Нина Ивановна, наш директор, была генералом. [И на Мальте будут русские расти (2002), «Домовой», 2002.03.04] ... наш Директор всегда была на посту. [Евгения Пищикова. Пятиэтажная Россия (2007), «Русская Жизнь», 2008]
We have a couple of words:
метро метрополитен фото фотография фотка ...
that denoting the same thing, but this words has different grammatical gender.
For us it is like a colour, that highlighting words in our text or speech.
We are denoting a gender of nouns, not things. We hear it like a musical rhythm that goes: bit-bum-boom-boom-bit-boom-bum-bum...
If we have it allready, we using it.
Директор была строгая. We mean she.
You can say
"Можга красивое," if you means it is
село, knowing that, I quote: Село Можга было возведено в ранг города.
Бостон may to be understood as the name of a circus troupe, ballet company, and so on. So we can say
Бостон выступала. We mean she.
Supposing a scene to be "house," and an author(speaker) had described it previously. The speaker might wish to point out some particular thing. He(or she?) would then have to say "a red apple," or "a red tomato", or "a red ball-pen". One might wish to allude to the quantity, one tomato, or two pen. Or to describe the kind of thing, as "a litlle tomato."; or to the serial order, as "the first thing,' or "the second thing."
Without ambiguity he can say give me a red litlle-thing. In Russian, every name of things has gender. Apple is neuter, tomato is masculine, ball-pen is feminine. So, we additionally can say: give me the red-masculine. Without any ambiguity.
Occurring in many inflected languages, in Russian too, Adjective, Noun, Verb, Pronoun has its own characteristic such as time, gender, case, persons and so on.
Imagine how we seeing and hearing highlighted text or speech. Yes, the writer/speaker has to do much more work than reader/listener. Because Russian belongs to group synthetic languages.
What the default gender of a particular (even if it's a man-made word
Воркалось, хливкие шорьки
Пырялись по мове
И хрюкотали зелюки,
Как мюмзики в мове.
is answered in every Russian textbooks.