What we can say for sure is that medieval Christians believed that PIV sex + ejaculation was necessary for conception in...almost all cases, for the obvious reason.

They also definitely recognized that (again, in nearly all cases), menstruation ceased during pregnancy, so the red and brown and black ooze that you see (or not) during women's periods probably had something to do with pregnancy while still inside the body. In fact, recipes (medicines) to "restore the menses" seems to have been the code or euphemism for abortifacients...

Additionally, people would have known that women orgasming was not necessary. Also for obvious reasons.

Nevertheless: peasants in medieval Europe and the Near East did not write down their folk knowledge, and they sure weren't reading Aristotle. But from the academic medical tradition the Middle Ages inherited from late antiquity, we have some idea of what (primarily celibate, or "celibate", male) scholars and physicians believed.

There were two competing theories, one derived from Aristotle and one from Galen. In the Aristotelian view, menstrual "blood," while inside the uterus, was the future body of the baby. The "seed" in men's ejaculate brought "form and spirit" to the amorphous blood.

Galen, on the other hand, argued that women also contributed a "seed" just like men did. (Don't give him too much credit, though--he also thought that the penis was an inverted uterus, and he wasn't talking about gender confirmation surgery.)

Feminist scholars have pointed out that the Aristotelian theory turns women's bodies into basically a passive target, with men providing all of the power (form and spirit). Galenic medicine, on the other hand, affords an active biological function to both women and men.

Things got more complicated from there, of course. The very popular De secretic mulierum ("On the Secrets of Women"), an anonymous 12th century treatise, argues that the direction in which ejaculate enters a woman's uterus affects how the fetus develops. And so follows some of the most erotic language to come out of the European Middle Ages:

Their [mum and dad] food should be digested and its superfluities should be expelled; then after the middle of the night or before daybreak...the male outght to become erect and mix with her.

At this time, the woman must remain absolutely still lest the seed be divided and a monster [a baby with birth defects] be generated. The man should copulate gradually; he should not raise himself up very much but rather should remain touching the woman's chest so that air cannot enter the lower members and prevent conception by corrupting the seed.

After ejaculation the man ought to lie on top of the woman without moving for about an hour, so that the seminal matter does not scatter and form a monster...

After this, the woman should extend her legs so that the seminal matter can spread itself out...Then she should lie quietly and sleep.

But for some reason, these theories still don't cover that one case very, very particular to Christianity.