The question in your wife's case is: is she doing this on purpose, or is this behavior out of her control? If this is something truly out of control, but she is trying to hide the fact, then she needs to consider sex therapy
or couples therapy for it. There is also the issue about the method of stimulation to bring her to orgasm. Many women find that they orgasm more easily by external clitoral stimulation than by internal G-spot
stimulation, or vice-versa. Some women climax well with either modality. If she is embarrassed to "ejaculate" you need to encourage her that her urine and vaginal fluids are not a turn-off to you and that you would love nothing more than to see her (and hear her) climax and release with a nice full-bodied orgasm even if your mouth is "in the line of fire" so to speak. She might be afraid to upset you by discharging a mix of fluids into your oral cavity when she climaxes. So set her mind at ease and let her know that you want her to relax and enjoy the process and not be afraid to finish and let herself go. Good communication is often key to the solution of these problems.