I say, and with God's success: It does not invalidate their I'tikaf; because he has no control over it, as it is neither sexual intercourse nor akin to it. If he can wash in the mosque without contaminating it, there is no problem with that; otherwise, he should go out, wash, and return to the mosque to complete his I'tikaf. Refer to: Bada'i' al-Sana'i' 2: 108, and God knows best.