I'm almost certainly misreading something here. However:
I was reading Insights to the Daf for Yevamos 34, and in response to a question that implies a Misnha on 33b contradicts a Gemara that says that a woman cannot get pregnant the first time she has relations (which at least today is not the case, but regardless), Tosfos HaRosh seems to say that the case of the Misnhah is where the girl was married before age 3 and had relations then, and therefore the Misnhah still considers the first intercourse after age 3 to be the first time, yet then she is capable of getting pregnant.
The TOSFOS HA'ROSH answers that according to the Girsa of Tosfos (see previous Insight) the answer is obvious. Tosfos explains that the Mishnah refers to a case in which the women became married before they were three years old. Any act of relations performed prior to that age did not constitute relations and the girls are not considered Be'ulos (since their Besulim return until the age of three). Consequently, the act of relations with their husbands after they reach the age of three is considered "Bi'ah Rishonah." (See also RASHASH.)
Now, I don't totally follow what he's saying, since the Misnhah in question says that minors cannot get pregnant, so it implies that the women getting married must be adults for them to get pregnant. Does it mean that they got married before age 3, had relations, got divorced, and this is their second marrage when they're adults?
Again I think I'm probably just tired and misreading something. But if I'm not, is Tosfos HaRosh really saying that the Misnhah is saying that girls under three years of age used to get married and have sex (be it in the time of the Mishnah or in the time of Tosfos HaRosh)? Or is it just a hypothetical thing?