The climaxing is the specifically sinful act, yes. In theory, provided there is no danger of climax, one could indulge in this prior to normal intercourse. However, I myself personally have a problem with the entirety of the act, since I think it pseudo-homosexual on its face.
I guess you have a problem with your wife using her hands on your, too? That certainly carries a risk for climax. By your definition of homosexual behavior, a woman using her hands on you is no different from a guy doing.
Do you and you wife engage in this homosexual behavior?
In addition, a woman's orgasm contributes to impregnation by distorting the cervix so that it reaches the expected seminal fluid. By giving your woman an orgasm without having an ejaculation, you are defeating this function. Anyway, on its face, the entire oral sex act with a woman is no different than lesbian behavior since no pregnancy will result.