I suppose there are guys who, like you said, either prefer oral as a fetish or preference, or think it's not as "bad" or "wrong" in terms of "betraying" an SO. However, I find many a time, my oral only sessions, while I have some clients who distinctly prefer it, more often than not it just turns out that way by chance, as we're going with the flow, and, for example, it feels good and we end up doing BJTC instead of switching from a bit or oral to intercourse. Or if we're doing 69, we both end up finishing, and we don't get around to a second round, either because it's a short appointment, or because we have other things planned- the gent wants to talk about something, or go out to eat, and/or is a one-pop guy. Often, it's regulars with whom I have had intercourse, and we both know it's on the menu, but there are those appointments when oral just feels so good that we both get distracted and forget to incorporate FS, lol. But I'm sure there are different reasons guys visit oral-specialists, vs. reasons guys might choose to visit me, knowing FS is on the menu, and then we just don't get to it in a specific session. I have to agree with meowbaby, I find that mutual oral can be just as satisfying without the intercourse- don't get me wrong, I love sex, but if a guy pleasures me orally I don't usually feel disappointed if he doesn't follow that with penetration, although it's always fun, it's not any less satisfying for me at least, to do a session that turns out to be an oral/manual only session. There are so many aspects- the touching, the kissing, the friction, the connection, the orgasms, that don't require FS to be a "full" sensual experience, IMO. But on the other hand, that's the nice thing about this hobby, for both myself and my clients- variety. I get to participate in many different kinds of sex, and meet many different kinds of people, whereas for the gents, they can pursue what they're in the mood for, or just go with the flow, and usually both leave happy! 
XoXo
Marea