There have been times when Mac users have been treated like red-headed stepchildren when it comes to the world of online gambling. Those times are changing a bit, but that doesn't mean it is happening all that rapidly. Still, if you have a Mac, you won't get shut out of playing.
Those people using a MacIntosh computer rather than a PC were at an extreme disadvantage at one time, because there was virtually no place for them to play at all. The reason that was given by Mac enthusiasts involved the fact that the hardware, and consequently the software, that worked with Macs was of such high quality compared to PC's that there was a problem with compatibility with online casinos, who are trying to make something that is compatible with the widest range of people possible. As a result, there hasn't really been a major rollout of casino software from casinos that has fit smoothly with a Mac. In fact, almost all of the online casino operators developed their software exclusively for Windows. So the Mac / Apple people had been more or less left out in the cold.
Things changed a bit, however, with the advent of the instant play casino. These contain games that do not require a download to play, and so naturally they are going to be more compatible with Macs to begin with. Because of Flash technology that is designed to be Mac-compatible, users generally don't experience any difference between playing these games with a PC or a Mac, in terms of graphics or continuity of the games. So that provides something of an "equalizer." And the Mac OS X user will not get cheated this way. Whenever you look at the casinos that have a download and no-download version, while it is true that the download casino generally has more games, the Flash versions are becoming more and more robust.
Poker has been a big problem in the past. There are still not many poker suites that have been developed to accommodate a Mac user. If you shop, however, you are going to find some of them. And when you do that, you will also find some of them that work with apps on your iPhone.
Some of the Mac access has to do with the rationale on the part of casinos that make the conscious decision as to what to do. Some may have a philosophy where they are going to look upon a Mac user in a certain way, and perhaps demographically that might be a desirable audience. But one must consider that there is different engineering involved in designing the software for the Mac, and that is generally a decision that is made by the online casino software company itself.