The argument here is that altruistic love, as an evolutionary advantage, is “bounded rationality”; stupidity. Wanting to protect the weak, especially when they are strangers, simply has no cause on the evolutionary perspective. No one raises a doubt about whether altruistic love can be explained by evolution (as, it can) but the Christian merely want to point out that it is either an agreement to use somebody or some dumb misfiring of tribal mentalities.
And the atheist merely expresses offence at the idea.
Who cares if love can be rationally bound and intelligently defended? It probably can’t. Love is blind and stupid and moronic. And brilliant and exciting and wonderful and powerful and empowering and humbling and exhilarating and preposterous and incredible. Anyone who has ever been in love knows the passion of having that one person at the very centre of their being. Anyone who has ever loved a cause knows the endless importance of this one collection of ideas. Love is an experience (and quite possibly the very best experience). And experience is justification in of itself. There’s no reason to apply evolutionary reductionism or even biological reductionism to love. You just sit back and revel in it. Enjoy it. Experience it. Work for the very purpose of maintaining and developing love.