Why stop at a rainbow triangle. Surely everyone should have the biological sign for male and female on their desks to show they dont discrimnate against persons of either sex. A Cross to show they don't discriminate against Christians, a Star of David to show they dont discriminate against Jews. A black power salute to show no discrimination against blacks. Etc etc. And lastly a Royal Bank of Canada corporate logo to show that the work area is a safe place for idiots.
... stick of dynamite to show they don't discriminate against Muslims...
Displays of any card or symbol displaying allegiance, support or tolerance for a particular cause is fine with me if it is a matter of personal choice. Happens all the time - flags, ribbons, etc. - and is supported by companies all the time (e.g., selling daffodils for cancer, etc.)
Safe space is explicitly voluntarily and a matter of individual choice. Your boss cannot force you to display the card - nor can your boss force you not to. If you can't agree in principle with what the card says, don't display it.
If someone asks you why either way, you're free to give whatever explanation you wish - they're free to question your beliefs as you are free to hold them. If you find that you can't defend your beliefs, perhaps it's time to consider why you believe what you believe.
If you get blacklisted or fired for not agreeing with any display of allegiance, you have good cause to fight back. This applies to all displays of allegiance, though - from a rainbow triangle to the American flag. Forced displays of false allegiance is fascist, regardless of the symbol.