A Versatile is a gay man who likes to be both active and passive in bed during sex. A synonym is "switcher".
The Versatile is generally considered to be a varied sex partner. Because: in bed he is both active as well as passive is on the go. Or in other words: he fucks and also likes to be fucked. Many gays are open about their existence as versatiles and thus "advertise" their sexual abilities in chats, among other things.
They are often very attractive to many gays because of the possibilities they offer. Accordingly, a relationship with a versatile is associated with many surprises (and position changes) and can expand one's own erotic horizons.
This is especially true if BOTH partners have corresponding inclinations and thus complement each other optimally. Because: it is of course also important to be able to deal with the flexibility of the versatile. Otherwise there is a danger that it will quickly become boring in bed - at least for one party.
By the way, many gays develop into versatiles in the course of their lives (and with increasing experience) by trying out more and more and at some point realising that their sexual interests are diversified. Versatiles are also - understandably - usually welcome guests at orgies and sex clubs.
But: of course, the statement "Versatiles are hard to please and unfaithful per se!" is a prejudice. Being versatile simply means being able to rediscover oneself and one's body over and over again - either with a steady boyfriend or man or with changing partners.