Men have to worry about pregnancy too. If a man gets a woman pregnant, it's ultimately her choice whether to keep the baby or not, so the risk is she would want to keep it and you gotta help raise the kid or pay child support.
It sounds like you're coming from a perspective of women only having one night stands. In a loving long term relationship, I hope that the answer to questions like "does this guy have a wife", "is he going to hurt me" or "is he just using me" would all be no.