Medieval sources actually tell us quite a lot about how medieval people thought about sex, if not exactly providing videotaped evidence of occurrences. The most important thing to keep in mind is that medieval Latin Christians categorized sex differently than we do today, including the concept of "positions."

For medieval Christians, with the ideals coming from Church writers first and foremost (contemporary medical authorities were also clerics, although they drew heavily on the work of pagan Greek and Muslim Arab and Persian authors), the most important classification for sex was whether it was with or against nature--that is, could it lead to repoduction under the right circumstances. Thus, PIV sex after menopause was still okay, because God could work a miracle. And so forth. This leads to interesting quirks of canon law like masturbation being technically a worse sin than adultery, although canon lawyers and pastors strive mightily to write their way out of that one.

So it is in light of that ideal that we get our first set of evidence for other sexual practices. Penitentials, basically lists of sins that offered guidance to priests in what they might see in confession, are sometimes quite explicit on what those "against nature" variations can be. Well, explicit in the way you might expect medieval celibates to be explicit. Thus, they give graphic detail about monks who hollow out wood to serve as, er, a receptor; but (presumably) anal sex is only "sex as the sodomites do." Note that this is prohibited for male-female partners, not just men--the issue, at least until the late Middle Ages when sodomy becomes an enforced capital crime of same-sex sexual activity, is the lack of reproductive possibility.

There is also a surprisingly rich tradition of condemning of intercrural sex, or rubbing against the thighs. Scholars have generally posited this as a means of birth control.

Medical texts give us further insight, if indirectly. A common theme in texts like On the Secrets of Women is the medieval medical belief, grounded in classical authority, that improper reproductive sex acts are the cause of birth defects! This can mean anything from not-missionary to sex at the wrong time of day to merely enjoying sex a little too much, especially if it involves moving around.

As far as lesbian sex, we have incredibly little evidence. For the most part, clerics refuse to even mention it, worrying that they might spoil innocent little female minds. (No, really). It's very evident that women were masturbating with dildos, of course. The infamous case of a woman prostitute executed for sodomy in 1477, Katharina Hetzeldorfer, reports that she used "an instrument" to have sex with her clients "as a man does." From the Islamic world, however, there is one famous and wonderful discussion of lesbian sex! It highlights the mutual nature of rubbing/friction leading to pleasure, with the lovely euphemism of the "saffron massage."

Edit: By popular demand...

The terms for "lesbian" and "lesbian activity" that show up in medieval Arabic sources (like sahq, sahiqa) are etymologically related to the words for rub or friction. Male medical writers, who recognized that some women were attracted to other women, attributed it to an "itchiness" caused by an imbalance of humors and temperature. Yuhanna ibn Masawayh, surprisingly for the time, even points out that lesbian sexual preference endures for a lifetime once it occurs (although this is a long way from how we think of sexual orientation as an "identity" today). Because the Middle Ages are the best ages, (dude) al-Kindi in the ninth century even thinks it is relevant to mention that women's bodies can and do produce liquid lube of their own during (lesbian) sex.

But the "saffron massage" passage comes a bit later, from 13C North African writer Shihab al-Din Ahmad al-Tifashi. There is actually a rich tradition of Arabic erotic writing, mostly in a very high literary vein. Texts like the Encyclopedia of Pleasure and The Book of the Dark One and the Gift from God mix information and stories, including about same-sex love and, well, sex. al-Tifashi's A Promenade of Hearts falls into this category. I'll borrow Saher Amer's English translation here:

The tradition between women in the game of love necessitates that the lover places herself above and the beloved underneath...This is how they act: the one that must stay underneath lies on her back, stretches out one leg and bends the other while leaning slightly to the side, therefore offering her opening (vagina) wide open: meanwhile, the other lodges her bent leg in her groin, puts the lips of her vagina between the lips that are offered for her, and begins to rub the vagina of her companion in an up and down, and down and up, movement that jerks the whole body.

This operation is dubbed “the saffron massage” because this is precisely how one grinds saffron on the cloth when dyeing it.

The operation must focus each time on one lip in particular...she does not stop acting in this manner until her desires and those of her partner are fulfilled. I assure you that it is absolutely useless to try to press the two lips together at the same time, because the area from which pleasure comes would then not be exposed. Finally, let us note that in this game the two partners may be aided by a little willow oil, scented with musk.

In contrast, the male Christian author of the Ancrenne Wisse, an early 13C (so, contemporary) text written to women, explains that he will say no such thing

for fear one should learn of more evil than she already knows, and should thus be tempted.

A couple of archived threads contain a lot of good information.

From February 2014, this one here has a top-level comment about medieval Europe and a lot of further information in the comments below.

And then this one covers Rome, South American cultures, China, Japan, ancient Greece, and ancient Egypt.

There's also one discussing Roman spintriae, which are coins that depicted various sexual positions and acts. I'm having difficulty finding that exact thread though.