Why didn't you call him out at the time? That's not a dig; I genuinely wonder why. Is he a friend? More influential than you? Did others seem to be agreeing with him?

Report it as you would any other incident of workplace harassment -- e.g., against "the women" instead of "the gays".

if you are saddened and bothered by it you have an obligation to do something about it. The next feelings are resentment and hate if anyone who is gay hears it. We dont want to spread hate forever.