There is a modern tradition of Antonio and Bassanio, and the rest of The Merchant of Venice, being interpreted in queer terms.
A common starting point for this is the first line of the play. "In sooth, I know not why I am so sad", says Antonio. We don't either. He's rich, he has close friends, he seems to have no desire to marry. He goes to great lengths for his friend Bassanio though, putting his wealth and even life at risk. At the end of the play Antonio is still making extravagant promises on Bassanio's behalf, now to his wife: "My soul upon the forfeit, that your lord / Will never more break faith advisedly".
It's all a bit much.
There's also a serve of cross-dressing, if you remember, with Portia and Nerissa dressing as men so Portia can be the lawyer for Antonio, and jokes about Bassanio sleeping with the (male) lawyer she pretends to be. There's plenty of other secret identities, concealments and revelations throughout the play, too: like Jessica converting to Christianity.
An early reference for the queer tradition is WH Auden's essay Brothers and Others, from 1948, which notes Antonio's obsession with Bassanio, and the Renaissance association of sodomy with usury. "It can, therefore, hardly be an accident that Shylock the usurer has as his antagonist a man whose emotional life, though his conduct may be chaste, is concentrated upon a member of his own sex."
The RSC website has a 1965 production with "the unmistakeable suggestion of a homosexual dimension to the relationship between Bassanio and Antonio". I wouldn't be surprised if a less institutional company got there even earlier. The Michael Radford film starring Al Pacino, also uses the text to make Antonio and Bassanio's relationship ambiguous. It could be showing them as gay or bisexual, or it could just be showing the difference in male friendships across different places and times.
More direct discussion of queerness - or just biting the bullet and showing the characters as gay - seems to start in the late 1970s, going by Joseph Pequiney's The Two Antonios and Same-Sex Love in Twelfth Night and The Merchant of Venice (PDF). This 1999 journal article by Steve Patterson goes so far as to describe it as a "modern cliche".
Lastly there's an excellent poem on the queerness of Merchant and the openness of Shakespeare in general to interpretation, The Sadness of Antonio, by Jason Schneiderman.