If the opposite were true, the human race wouldn't have lasted this long. Most of us are the result of individual acts of human love.
Even though I agree somewhat with that. There are bacteria and viruses and other very "loveless" life forms which are much older then the human race and will most likely outlive us without the need for love,
Plus, if it truly was "human nature for love to win" we would live in Utopia and not in Reality land were people kill out of joy and greed. We'd just all love each other which is clearly not the case in the real world.
The progress you described is nothing new either. Many empires had more open minded views on sexuality and love then the modern western world. Look at India today and you'll see a bunch of sick shit on every corner. The same goes for China.
Much as interracial love was once taboo and is now largely celebrated the world over
Interracial love is nowhere "celebrated" let alone all over the world. Maybe in some hipster cities in the US and Europe but the rest of the world hates that shit.
homosexual and even polygamous or open love will eventually be accepted as normal and the homophobes will go down in history as the misguided bad guys.
That's just wishfull thinking on your part.