If having multiple wives is Sunnah, why are women against it? Is Western cultural influence greater than religion?

So the first thing is that it's not specifically sunnah to have more than one wife according to the majority of scholars. Yes it's permissible islamically and we have to make a difference between something that is permissible and something that is specifically sunnah. Something that is sunnah has to fulfill certain criteria and here according to the majority of scholars for example Ibn Qaddafi mentioned it in the Hanbali Madhab, Imam Shafi'i mentions it even in the Shafi'i Madhab, Imam Qaddafi mentions it, Shirbini mentions it as well, that it's not specifically sunnah although it would be permissible. Now in terms of the second part of your question then with regards to that I'm sure many females may be able to answer this better and put their perspective in the comments but maybe it's to do with the legal reasoning in the country that you're living in according to the laws that may not be permissible for example. Maybe it's concerned about a financial aspect and also from the male perspective one has to be, if one does take that route, one has to be able to be just and all the aspects that a male would be required to be.