Says who? While I could see an alternate
Loving leading to SCOTUS ruling in favor of gay marriage earlier, having the court legalize gay marriage in
Loving itself does not seem at all plausible to me.
I'm not sure how much it would change the progress of gay rights movement. It would delay the legalization of gay marriage, but not necessarily by all that much--assuming public acceptance progresses as OTL, by 2017 there would be
44 states with a majority in favor of gay marriage, which is probably enough to get repeal put through by TTL's 2020s, if not earlier.