Bond/bound/binding - all key words and key concepts in "The Merchant of Venice". And by comparing Bassanio's double bond, to Portia and Antonio, you draw out the unusual financial/emotional natures of these transactions. What Shakespeare seems to be saying is that you cannot separate the two.
I'll show you what I mean. Firstly, Bassanio is bound to Antonio because Antonio has entered into a legal bond with Shylock in order to get the money for Bassanio to go off to Belmont:
Three thousand ducats for three months and Antonio bound.
Yet there is also a strong emotional connection between the two. Antonio is "sad", we find out in the first scene, and many critics have argued that it is because he is in love with Bassanio (with whom the other characters then leave him at the end of the scene somewhat conspicuously!). This "bond" is much more than just money.
Then, in Belmont, Bassanio enters into an emotional, but also a legal bond: marriage, sealed by a ring (which becomes the symbol of the bond):
But when this ring
Parts from this finger, then parts life from hence:
O, then be bold to say Bassanio's dead!
Marraige is supposed to be primarily emotional, and secondarily legal. Antonio's bond with Bassanio is supposed to be primarily legal and secondarily emotional. But it doesn't work like that:
Antonio, I am married to a wife
Which is as dear to me as life itself;
But life itself, my wife, and all the world,
Are not with me esteem'd above thy life:
I would lose all, ay, sacrifice them all
Here to this devil, to deliver you.
Your wife would give you little thanks for that,
If she were by, to hear you make the offer.
Bassanio, as he does throughout, uses the language of value. He "esteem"s his own life, his wife, and the world not as highly as he does Antonio's life. So his bond - supposedly just legal - crosses over into his marital life, and reveals that his "legal" bond is esteemed more highly than his wife. This is ... to say the least, problematic, as Portia points out.
So Antonio seems to be the character Bassanio is most bound to. Though it should be Portia. And this uneasy menage-a-trois is not really resolved, even as the play comes to its close.