Yes, it was. In fact it remained illegal until 1967. In that year, after recommendations from the Wolfenden Report, private acts between two adult men were decriminalised ('adult' meant over 21, though the age of consent for the rest of the population was 16).
Even after 1967, the new law did not apply to the armed forces or to Scotland. It didn't result in a much more liberal climate for gay men; in fact there is some evidence that there may have been more prosecutions after the 1967 Act than before it, as in some ways it was easier to identify when the law had been broken - such as by public acts, or gay bars.

The age of consent for gay men wasn't reduced to 18 until 1994, and full equality was finally achieved in 2000 with the age of consent lowered to 16 for everyone.

You can learn more about the history of gay rights here.