First of all, you aren't being "strongarmed" to pay dues or forced to be in a union. You knew you would be in a union when you accepted the offer and if you had such a problem with the idea, you could've sought work elsewhere.
Also, it is unfair to say that you could negotiate equally good compensation. First of all, you probably couldn't (the data bears this out: unionized workers do better than their non-unionized counterparts) and second, if you could, it would only be because the union has already successfully established a baseline compensation package your position through collective bargaining. Workers at Toyota factories in the US benefit from the work of their union brethren at Ford, GM and Chrysler.
You can't opt out of the union because, as others have noted, the union gains its leverage by controlling the labour supply. If you opted out, you would not only damage the union's position but you'd also be receiving the benefits of the union's work (higher wages and benefits, job security, grievance protocols, etc.) without paying dues.
Also, in general, the longer you stay at a job, the more visible the benefits of union membership become. If you've only recently started, its unlikely you've needed to file a grievance or take paternity leave or draw on pension benefits or experienced a pay cut, a wage freeze or a round of layoffs. If you've only been there a few months, its easy to only see the garnished wages and get indignant.
On a personal note, I was in a union when I was a teaching assistant in graduate school. I was paid more than $35 an hour which was much more than my friends who were TAs at non-unionized schools earned. Maybe your union just sucks but presumably a majority of your colleagues support the union -- other your local would vote to disband.