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Yes, absolutely. A guy can even ejaculate without being hard. That is the reason you never have a guy inside of you period unless you are protected in some way.
According to the Quodoushka tradition there are different types of orgasms and ejaculations and one of them includes being soft. I think it is pretty rare in our society because we typically don't think we are able to and probably stop, but I have had a soft-on and ejaculated. It wasn't completely soft, but wouldn't have been considered hard either.1