Before Puberty
The most obvious example is the orgasm of boys before puberty. Since their internal sex organs are not yet sufficiently developed to produce semen, there is nothing that could be ejaculated. Nevertheless, these boys can have orgasms.

Supply Exhausted
Certain other men who are capable of several orgasms within a short time may, for a while, exhaust their supply of semen and thus stop ejaculating as they continue to have orgasms.