He didn't and he can't and won't.
Get the thing in context. Pope Francis was talking in an informal - and probably imprudent - way in a film celebrating his life and work. Bit of a vanity thing, really.
He was obviously trying to show himself in a good light, and emphasising that in opposing same-sex marriage he wasn't motivated by spite. He ended up giving a powerful message which he should have seen would be used by all who oppose the Church's teaching.
He should not have chattered on about this subject. I fear he allowed his desire to look trendy to overrule common sense, prudence and a sense of duty.
A lot of damage has been done, and it may not be within his power to undo it.