I don't think the party should have anything to do with it. I would give no benefit of the doubt to someone who is revealed to have committed harassment resigning to resign because some member of the other party who also was guilty didn't resign. Anyone who does this should resign.
Franken not doing so should be an embarrassment to every member of his party as well as every member of any other party.
That's a fantastic ideal to have, but the most realistic way to make that ideal reality is for both parties (and their voters) to care a lot.
I'm not opposed to him resigning. I do think proportionality matters though, and what Franken did (that we know of) pales in comparison to what others are accussed of. However, I am very much ok with drawing the line on the side that said what he's allegedly done is enough to have someone resign. Of course how we decide the validity of allegations is a crucial component as well.