Should this infamous question have a sexism tag or not?
This is a discussion easily avoided by simply not including the foot note at the bottom of the answer (it is now gone).
Semantically I don't believe the question is sexist at all - making a comment relating to a biological function of a biological gender in itself isn't sexist, although the comment or assertion can still be factually wrong.
In any case no tag is needed because the question wasn't explicitly about sexism, and any sexism was perceived or inferred.
It depends on clarification from the OP just what they are asking about, some interpretations of the question may touch on sexism, but not necessarily so.
By my initial reading, the question has two parts, but overall has nothing to do with sexism:
1. Is a particular practice important [to be effective in martial arts in general]?
2. Will practicing it [harm] my relationship with my wife?
With this reading part 1 is a yes or no question sexism does not come in to play. The practice somehow actually has a positive impact on the martial artist's performance, or it does not. If it works, it isn't sexist (although the perceived reason for why it works may be.) If it somehow only worked for men specifically because semen were magical, it still isn't sexist, no matter how unfortunate it may be that this route to power were only available to men.
Part 2 is also a yes/no question, but no one besides his wife can can give a definite answer to. Different people have different appetites and needs. One person may be happy to go along with the self imposed abstinence for a period of time, and another may not.
On followup, I see an alternate reading the question that still has two parts and sexism is central.
1. Specifically in the context of this martial art, is the practice important [to be an effective member of the community fostered by this art]?
2. Will participating harm my relationship with my wife? (still a question for the wife, not us)
With this interpretation, the inclusion of the sexism tag is entirely appropriate if the community's behavior and teachings regarding the practice are themselves sexist. If the community behaves as Sardathrion describes in their answer, then yes include the tag because they are asking "Do I have to participate in this sexist behavior to be taken seriously by my peers?", and "If I adopt this sexist behavior, will my wife like it?"(still a question for the wife.)