Given that some are discussing building and running some comparisons, I return to this statement for a moment:
In any case, this does not pertain to the monk issue - any number of other classes can become a kensai; the PrC is not specifically for monks.
I did some research, going through a pile of WotC books* as well as Dragon and Hyperconscious (2 3rd party sources I commonly use) in order to look at Monk/Monk-friendly PrCls, and found 23 of note. Those from 3rd party sources are boldfaced w/ source listed.
Sacred Fist
Tattooed Monk (OA & CompWar are essentially the same)
Kensai
Fist of the Forest
Cloud Anchorite
Shadow Sun Ninja
Enlightened Fist
Monk of the Long Death
Disciple of the Word
Shen (Dragon #319 p68)
Monk of the Enabled Hand (DCv1 p68)
Arcanopath Monk (DCv1 p80)
Henshin Mystic
Drunken Master
Fist of Zuoken
Zerth Cenobite
Shintao Monk
Lucid Cenobite (Hyperconcious p140)
Reaping Mauler
Frostrager
Master of Nine
Incarnum Blade
Spinemeld Warrior
Of the 23 PrCls listed, some freely multiclass with Monks, others don't (and the names are NOT indicative). NO class on the list of is limited by having "Monk X lvl."
Of those 23 PrCls, only these:
Henshin Mystic
Drunken Master
Fist of Zuoken
Zerth Cenobite
Shintao Monk
Lucid Cenobite
have access limited by a class-specific requirement common to Monks. All others are open to all other class combinations capable of meeting a BAB, Skill & Feat combination.
1) Of that short list, the Henshin Mystic has access limited by having "Purity of Body," a Monk ability functionally identical to the Paladin's "Divine Health." Theoretically, that means a Paladin could qualify for the class, depending upon your DM.
2) Of that short list, the Drunken Master has access limited by having "Flurry of Blows," an ability the Monk shares with the Sohei (see the OA class' Ki Frenzy ability, which combines a modified Rage with FoB for the PC's favored weapon), and the Battledancer from DCv1.
3) The last 4 on that short list have access limited by having "Still Mind," an ability which, AFAIK, is unique to the Monk, but I do note that there are some classes that have similar abilities due to their Auras etc., so there may be some classes that qualify that I don't know about.
IOW, there are very, very few- if any- PrCls that Monks qualify for that are barred to PCs without Monk levels.
* WotC sources examined included Core + Completes, PHB2, Unearthed Arcana, Tome of Magic, Bo9S, BoED, BoVD, Sandstorm, Frostburn, Savage Species, Planar Handbook, Races of Destiny/Stone/Wild, XPH, Oriental Adventures, Eberon, Dragonlance, Forgotten Realms, Player's Guide to Faerun, MoF, and MoI. I freely admit that I don't own every WotC book out there & I didn't go through all the ones I owned- if you know of a PrCl that is exclusive to Monks, please tell me where to look.)