|She's just a simple, innocent, peasant girl!|
Caveat: There are lots of great articles and in depth analysis available on this topic. These are just some quick notes to get you started if this is a theme of your writing.
While it may seem that social mobility in the Middle Ages was largely stagnant—with peasants being tied to the land and the noble class off-limits—this is largely misunderstood. Throughout all periods of the Middle Ages (Early, High and Late Middle Ages and into the Renaissance) status was more mobile than most people think. You might not become king through inheritance, but you could move up or down through the ranks. It could take a few generations, but rising and falling happened all the time.
Marriages of nobility were often for position and/or gain, regardless of gender and sometimes with little consideration to class. If a merchant was rich enough, influential enough, or both, a noble family, down on their luck, might well salivate at the chance of marrying into all that power and wealth. The reverse is also true, as a merchant class family might want to gain additional influence (and thus wealth) by achieving membership into the peerage, which offered certain additional advantages that couldn’t simply be bought.
Downton Abbey (although set in the early 20th century) offers a wonderful thumbnail sketch in just the first episode and throughout the series where the overarching focus is on saving the dying institution of the great estates. Initially, the problem was solved (before the show starts) when Lord Grantham specifically went to the United states to marry a rich American Cora, who had no title or nobility whatsoever. Her money was then tied, inextricably, to the Downton Abbey estate, with no hope of untying it when the initial heir died on the Titanic. The reverse was also true. The family was hopeful that the new heir would want to marry Lady Mary, and thus secure their joint futures.
|He's just a simple, innocent, peasant boy!|
Some social structures were far more rigid depending on the time and place. In Japan during the Sengoku Jidai, peasant to great lord was possible (though never easy). However, Hideyoshi's Social Status Control Edict or the Separation Edict enacted in the late 16th century IIRC changed all that, and made social mobility much more difficult. Peasants could no longer go to war, which pretty much tied them to the labor class permanently, and samurai were not allowed to farm or become merchants. This became even more structured during the Edo period in the early 17th century. There were still ways around this (like being adopted by a samurai family), but social mobility became much harder.
By contrast, during roughly the same period, Italian city-states were incredibly mobile social structures. Of course, it depends on how rich or influential (or both) the individual in question is, which could make for a powerful alliance (of give access to ready cash loans) depending. Most importantly, when writing about any of these time periods and your pairing of a peasant and a noble, you want to keep your social, political and technological standards as period-correct as possible.