The same thing is true about sex in general. If rape is bad because it risks causing unwanted pregnancy, then casual sex should be considered equally bad because it is just as likely to lead to the same outcome.
This is more of an argument against casual sex than against rape. Single mothers are a huge burden on the taxpayers, and there are millions of welfare single mothers in today's feminist society where rape is illegal.
You are assuming that the rapist would not be willing to raise the child at this own expense. Sometimes this is undoubtedly true, but there are also lots of non-rapists who want nothing to do with their children. It should be the responsibility of the father of the child to raise the child and provide for it. If the father refuses to do so, the child should not be supported at the taxpayers' expense.