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Question:

I know neshikas erva (kissing a woman’s sexual organ) is forbidden for a man, but is it forbidden according to the halacha for a women to do neshikas erva (oral sex) to a man, assuming there will be no spilling of semen? if yes, what is the source?

Answer:
Firstly, of course you are speaking about a husband and wife, because any activity of this sort is certainly forbidden for people who aren’t married. What you describe is sort of like playing Russian roulette with a loaded gun. Even a drop of emitted semen is zera l'batilla (spilling semen in vain) and how can you be sure that a drop won't be spilled during this type of arousal, especially as this can occur without any sensation in the man? In addition, the Arizal defines the clear fluid that is sometimes emitted prior to, or without ejaculation as zera l'batilla of an even higher level, deriving from a higher source. In his book Sefer HaZichronot, Parsha Bereshit, Rabbi Zadok HaKohen, mentions that some authorities forbid what you describe without citing their names and sources.