The best evidence I could find were the words which were spoken out of Portia's own mouth,
One half of me is yours, the other half yours--
Mine own, I would say; but if mine, then yours,
And so all yours. III.ii.16-18.
There you have it. Portia is Bassanio's. This may be idle lover's talk; however, it does signify the beginning of a possessive relationship in which Bassanio is the possessor!
Directly after Portia is "won" by Bassanio's correct choice in caskets she states,
Happiest of all, is that her gentle spirit
Commits itself to yours to be directed,
As from her lord, her governor, her king.
Myself, and what is mine, to you and yours
Is now converted. . . . I was the lord of
this fair mansion, master of my servants,
Queen o'er myself; and even now, but now…