True, we had never discussed exclusivity.
沒錯，我們從來沒有談過專一的問題。

But while for me, the idea of seeing another man would be like trying to fit another outfit into an already over-stuffed suitcase, Big was happily dating another woman like it was the most natural thing in the world.
不過對我來說，再跟另外的男人交往就像往一個滿滿的箱子里塞進更多的衣服。而大人物卻正在開心地跟別的女人交往。

Is it that men have an innate aversion to monogamy, or is it more than that?
是不是男人生來就不能專情，亦或是另有原因？

I couldn't help but wonder: in a city like New York, with its infinite possibilities, has monogamy become too much to expect?
我不禁想到：在紐約市這樣一個充滿無限可能的城市里，一夫一妻制是否是一種奢求？——《欲望都市》

can't help but... 不禁......；忍不住要做某事
help 在這里的意思是防止或避免，通常用于否定句型can't help，后面可以跟動詞原形或doing，表示忍不住要做某事、不禁......

例句：

Looking at the night sky, one can't help but wonder if there are other forms of intelligence out there.
望著夜空，任何人都會思考是否宇宙外空還有其它的智慧生物。

After seeing your performance on the last test, one can't help but wonder what studying you've been doing in the past three months.
看了你上次考試的表現后，讓我們不由得懷疑你在過去的三個月里究竟學了什么。

I can't help laughing every time I think of that.
每次一想起那件事，我就會忍不住笑起來。

I couldn't help noticing your ring, it's beautiful.
我沒法不注意到您的戒指，太漂亮了！