Don’t worry about any form of physical activity preventing implantation. It’s true that a number of different muscles contract during orgasm, some of them voluntary and some of them involuntary, but there’s nothing to suggest that this could reduce fertility.
Women who have multiple orgasms and women who indulge in excessive physical effort such as competitive sport (as long as they don’t train so hard that their periods stop) conceive at least as easily as women who don’t. Try not to let your anxiety about getting pregnant affect your sexual relationship with your husband. Even if you could prevent yourself from having an orgasm, I don’t think it’s a good idea.