I'll answer these each in turn - with a brief forewarning that I am dashing this off a bit, so consider this in-summation.

First, I assume with "knights and gentlemen" you are referring to all nobility short of a King or Prince. Which makes this a little tough as one's degree of authority and power, the lowest of nobility being a knight and duke's being near the high end would generally impact how on would navigate the politics of infidelity. Secondly, I need to inform you that the commonality of infidelity is tough to define -as regardless of its commonality it was generally considered 'polite' at the very least not to write or record it. Most recorded infidelity was likely done in an active attempt to discredit or villainize a certain character or disrupt the legitimacy of an "illegitimate" heir.

Commonality? Well, there were two sorts of infidelity, generally. That with a mistress (an unmarried woman) and that with a married woman.

Firstly, it must be noted that being a mistress was actually an accepted occupation for a young woman. And was even portrayed as quite an honorable one in ballads such as The Three Ravens (1611, original author unknown), it could be very profitable and was generally somewhat secure. If the mistress was particularly lucky she would eventually marry their lover or be recommend by her lover to an unmarried friend or acquaintance for marriage or to be their lover if ever it ceased to work between the two (yes this actually did happen more than you'd think. Ives Erik, Marrying for Love). And to answer the second question right now, as it seems to fit - one did not need to be nobility or hold a title to have a mistress. The only requirement to hold a mistress was to be able to afford one, oftentimes an easier task for a merchant than a knight - as a mistress could be quite expensive, especially since prostitutes have been available, and far cheaper, throughout history. So while it would be accepted, and perhaps even decently common, for a wealthy titleless man to have one (or more) mistresses, it may be a stretch to say it was "expected". Though it certainly would not have surprised, or likely offended, very many people at all. This commonality would likely extend to titled nobility - though they certainly had to do a better job keeping it a secret as they were more public figures and, in the middle ages, granted their right to rule through "divine right" and must therefore be pious. This would make matters of their fidelity a matter of interest to political rivals and religious officials - who both had a power stake in such issues. Then of course there were matters of the legitimacy of one's heirs to consider - for if one had too many children by a mistress the matter of potential pretenders and instability would come up..

There are a lot of fun, explicit... sometimes very dark... and by all accounts representative stories about mistresses, prostitutes and infidelity to be found in the Canterbury Tales by Chaucer. These provide a fantastic and deeply interesting representation of how these various sorts of sexual encounters were perceived at the time - one must not discount the power of fiction in presenting social mores.

As for the other sort of infidelity? Generally speaking the wife would hold the most blame, even if they were not a consenting party in infidelity relationship. And seeing as i just mentioned Canterbury tales - it is the wife in the Knight's tale that carries all the dishonor when she is merely approached by another man who wished to bed her. To the point where she contemplates suicide. The tale even mentions how this man in question, who approaches the wife, does not violate her purely to preserve her and her husbands honor - he expresses no concern for his own honor.

I am stretching this out a bit and i'm already a little late, if there are further questions after this brief summation i'll answer them when i get back. Infidelity among medieval nobility was common enough that it was often a topic of note in various accounts and an often adressed issue by the church - though as long as one kept it sufficiently private and under control it was generally accepted. Under control meaning that one kept this infidelity to mistresses and that one maintained a number of legitimate heirs along with any illegitimate ones. Other people's wife's were not an acceptable outlet, though it harmed the wife far more than the cheating man. Common prostitutes were also somewhat acceptable, even if married.

And the only barrier to having a mistress was that of wealth. A wealthy man, regardless of class, could have as many as he could afford - though not "expected" it was readily "accepted". And such infidelities were generally far easier to conduct as a wealthy commoner than they were as a noble -where matters of politics came into question.

What does "titeless" refer too?