先生 is an address with long history. But it is important that this address is only for male during a very very long time. Here are part of them (may have relevance to this question):
- Original meaning is literal, first born.
《诗·大雅·生民》：诞弥厥月，先生如达。 朱熹 集传：“先生，首生也。 Later extended as father or elder brother.
《仪礼·有司》：其先生之脀，折胁一。 郑玄 注：先生，长兄弟。
- Older and learned people, extended as teacher, professor.
- General address for scholars.
There are only a few females called as
先生. They share some common characteristics, like noble character, high prestige, well-educated and not young (middle or old age). And using such address on them is kind of respect. One with such title should be of great charisma, specialist in some field, famous and approved by common people. Additional, usually their husbands are of this kind too.
So when did this address begin to use for a female? Modern times. (Hint: the following parts may be a little subjective.)
After later Qing Dynasty, more and more western culture entered China, including feminism. When many females chose to be a teacher or even professor, the meaning of this address was extended.
But why they must be called as
先生 the only way to tell others they are respected? I don't think so. There may be still some shadow of male chauvinism.
And at last, use this address with great caution. It is strange to address a lady usually.