True, we had never discussed exclusivity.
没错，我们从来没有谈过专一的问题。

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.
不过对我来说，再跟另外的男人交往就像往一个满满的箱子里塞进更多的衣服。而大人物却正在开心地跟别的女人交往。

Is it that men have an innate aversion to monogamy, or is it more than that?
是不是男人生来就不能专情，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：在纽约市这样一个充满无限可能的城市里，一夫一妻制是否是一种奢求？——《欲望都市》

can't help but... 不禁......；忍不住要做某事
help 在这里的意思是防止或避免，通常用于否定句型can't help，后面可以跟动词原形或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.
望着夜空，任何人都会思考是否宇宙外空还有其它的智慧生物。

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.
看了你上次考试的表现后，让我们不由得怀疑你在过去的三个月里究竟学了什么。

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

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