Abortion is certainly a controversial issue, and a perfect topic for an ethics course. The question is not exactly asking for our opinion on abortion, however. The wording asks us if immoral should equal illegal, or that moral should equal legal. The specific answer to this question should be applied to ALL ethical issues involving law, not just abortion.
From a consequentialist / utilitarian perspective, the laws surrounding abortion should evolve based on what is best for most. Interesting enough, this means that there are instances when abortion should be legal, and when it should be illegal. The end result of whether we need more people is a matter of economics. For instance, if there is a massive surplus of people which causes more issues than it resolves, such as in the current era, then abortion should not only be legal, but encouraged in many cases. In this scenario, this should be a compliment of birth control of various means, as abortions are not cheap. If there is a shortage of human beings, it should be considered immoral and subsequently illegal to abort a baby, with the exception of cases of incest pregnancies- which has its own long term detriment to the greater good and should always be illegal.
Those in power decide what is lawful and not, even if those do not reflect what is right or wrong. In the case of the United States, it is for the most part the populace that decides. While we do not live in a society with 100% consequentialist laws, the current legislation roughly reflects how it should be, even if for the wrong reasons. Then again, do the wrong reasons matter as much as the result? That would be… inconsequential.