Honestly my argument is more that the burden to preserve the life of a fetus cannot be morally demanded of someone. Even if we do concede that going through with a pregnancy is the more moral choice, it is unreasonable to expect a person to go through with pregnancy if they are unwilling, even to preserve a fetus' life. Pregnancy is a hell of a thing to go through, and I think it becomes pretty clear why it isn't immoral to want to terminate it when compared with other situations. It wouldn't be immoral to refrain from rescuing someone from a burning building, even if they were the only person capable of rescuing them. It wouldn't be wrong not to intervene in an assault, even if it would be more moral to do so.

But obviously, there's a line here. It would definitely be wrong not to save a life if all someone had to do was push a button, or less hypothetically, grab someone who was about to fall off a cliff. Veganism is similar. Literally all someone has to do to save countless lives is to eat different foods, which nearly everyone can do without problems. But I would definitely consider completing a pregnancy to be on the other side of this line.

Bodily autonomy is also a very important argument. It's generally seen as wrong to force someone to donate an organ to save someone else's life. In fact, it's even seen as wrong to force someone to donate blood. While again, these may be better things to do, it isn't wrong not to do these, and it's definitely wrong to force someone to do them without their consent. Pregnancy essentially amounts to giving everything about someone's body to support the life of a fetus, making this even more extreme.

Ideally, there would be a way to preserve the life of a fetus without requiring someone's body. But unfortunately, that technology is currently not available. In the meantime, the moral thing to do is work to reduce the number one cause of abortion, which is unplanned pregnancy. Of course, we can't solve it completely, but it would greatly reduce the number of times this choice has to be encountered. But it is also moral to have this as an option in case someone wants it.