Last Thursday the human rights activist Peter Tatchell gatecrashed the opening night of the London Symphony Orchestra's Berlioz season at the Barbican to protest against Valery Gergiev, calmly walking onstage before a note had been played to make a speech denouncing the LSO's principal conductor's support for Russian President Vladimir Putin. "Gergiev is a great conductor," Tatchell said, "but he colludes with a tyrant and shows little respect for freedom and equality."

The tipping point for Tatchell, and also apparently for the organisation Queer Nation who interrupted a Gergiev performance at New York City's Carnegie Hall in early October, were comments Gergiev had made to a reporter in Rotterdam. Legislation Putin signed into law earlier this year banning the promotion of homosexual "propaganda" was, Gergiev said, about protecting his fellow countrymen from paedophilia, remarks that crossed a boundary by equating homosexuality with child abuse.

Were Gergiev's words merely dim-witted and ill chosen? If you're feeling charitable you might like to think so, but his quote makes for an unnerving read in the context of everything else Gergiev has said about Putin, and his point-blank refusal to speak up about other breaches of basic political freedoms in Russia. After members of the punk band Pussy Riot were imprisoned for staging an anti-Putin protest at the Cathedral of Christ the Saviour in Moscow in February 2012, Gergiev implied, bizarrely, that the band were motivated merely by increasingly their profile and "earning millions and millions". But consider this. In 2012 Gergiev, along with 549 other prominent Russians, signed a petition in support of Putin's re-election. And earlier this year, Mariinsky II, a state-of-the-art new opera house in St Petersburg, Gergiev's centre of operations in Russia opened – bankrolled by the state to the tune of a cool 200bn roubles (£450m).

Gergiev and the LSO's problem is that political actions have musical consequences. I've spent life-changing evenings at the Barbican listening to the LSO. Concerts by Leonard Bernstein, Dave Brubeck, Michael Tilson Thomas and Colin Davis will stay with me forever, but now the LSO brand feels tainted through association. The New York Times reported an audience member in Carnegie Hall yelled "This is an artistic event", as though distinguishing between Gergiev's political and artistic beliefs were possible. Personally, I'm not so sure. Stravinsky was on the programme that night. But had Gergiev been conducting the Pathétique Symphony by the gay Tchaikovsky, or practically anything by the endlessly persecuted Shostakovich, what then?

In my day-job reviewing CDs, I've given at least one glowing accolade to a disc of Gergiev conducting Shostakovich. But now I believe his credibility as an interpreter of Shostakovich and Tchaikovsky, the two composers around which his reputation is built, has been shot to pieces. Does Gergiev, like Vladimir Medinsky, Russia's culture minister, believe that Tchaikovsky wasn't gay but merely "a person without a family"? Even Putin was forced to backtrack on that Comical Ali nonsense. But if Tchaikovsky's homosexuality is accepted, how can Gergiev continue to conduct his music in Russia against the backdrop of repression by a regime he helps maintain? I wonder who Gergiev reckons Shostakovich would have supported: Putin or Pussy Riot? And I also wonder what Gergiev thinks all those nightmarish "chase" motifs are about in Shostakovich symphonies? His get-out clause: this is absolute, abstract music. But if Gergiev sincerely believes that, I'm not certain I want to hear his interpretations again. If not, then he's engaging in dishonest, desperate self-justification.

Tomorrow night at the Barbican there will be another protest; yesterday the orchestra were forced to issue a statement on twitter (since, apparently, deleted) suggesting "Gergiev's views are his own, and not those of the LSO". Earlier this afternoon he issued a statement in which he denied that "I have ever supported anti-gay legislation"; but a man who has failed to distance himself from Putin while carelessly stigmatising the gay community can't wriggle out of it that easily. And nor do such sentiments as "I am an artist … I collaborate with and support all my colleagues in the endeavour for music and art" wash; support for Putin and sympathy with Shostakovich are incompatible. And, truth is, the mood music is turning against Gergiev, and his position is starting to look untenable.

It's good to know, though, that Putin's anti-gay legislation is backfiring. I'm a 41-year-old happily married man with one child, another on the way, and I've never been on a protest in my life. But tomorrow I'll be at that Barbican protest. Gergiev, honoured by Putin as a Hero of Labour, now has another badge of honour: he's turned me into a gay rights activist.