No, we won't lose 2.3 million jobs
The CBO projects that the act will reduce the supply of labor, not the availability of jobs. There's a big difference. In fact, it suggests that aggregate demand for labor (that is, the number of jobs) will increase, not decrease; but that many workers or would-be workers will be prompted by the ACA to leave the labor force, many of them voluntarily.
As economist Dean Baker points out, this is, in fact, a beneficial effect of the law, and a sign that it will achieve an important goal. It helps "older workers with serious health conditions who are working now because this is the only way to get health insurance. And (one for the family-values crowd) many young mothers who return to work earlier than they would like because they need health insurance. This is a huge plus."
That said, some non-fulltime workers will indeed have hours cut -- non-voluntarily. I know one person to whom that is happening. Yes, that pisses me off. Yes, I wish we had gone another way. But 13 million more people will have health insurance, and that's important.
By the way: If there's any truth to this whole notion of "supply and demand," then shouldn't reducing the supply of labor lead to higher wages? Just asking.