There is a relationship in the MicrosoftCrmPortalDependencies managed solution. It is a 1:N relationship from contact to opportunity named msa_contact_opportunity. It has the following field mapped: contactid from the contact entity to the msa_partneroppid field on Opportunity. The display name of the field on opportunity is "Partner Contact". It doesn't make sense to populate the "Partner Contact" on an opportunity every time an opportunity is created from a Contact. Many opportunities have nothing to do with the Partner Portal. The Partner Contact field on an opportunity should only be filled in with a portal user if it's an opportunity that is being passed over to a portal user to work on it.