If one occurs the guy has zero choice or say about what follows. If she decides to have the child it doesn't matter what he wants one or not, he's automatically now a father and can be pursued for child support.
Over 18 years this adds up to an average of just over $100,000 so two years of the average salary. That's two years of his life spending his days doing a job to pay for a child he never wanted, delaying retirement, potentially preventing home ownership, and significantly impacting his ability to travel or otherwise make use of his income.
Even if he uses a condom, or gets a vasectomy, or she's on the pill an unwanted pregnancy can still occur. Condoms can break, pills can fail or be forgotten, and even vasectomies can fail. The chance may be small but it is life-changing when it occurs, and he's utterly powerless to do anything about it when it does.