Maria the Copt

Most scholars do not consider Maria as one of the Prophet’s (ﷺ) wives.

But isn’t there a ruling in Islam which says a man has to marry his slave when she bears him a child?

That is not the ruling regarding a slave who bears a child for her master. The ruling for that is that once she gives birth to a child from him, even if the child dies, she is to be freed upon the death of the master. Also, she is referred to as umm al-walad which distinguishes her from other slave-women.

And Allah knows best.