No, marriage doesn't involve children when a 50 year old marries another 50 year old. But the institution of marriage, and why society cares about it, and why government thinks it is appropriate to regulate, IS about children.
It isn't about love. It isn't about feeling. It isn't about wants. It has nothing to do with you or me, as individuals.
It is about what works well, when followed by SOCIETY - get away from individuals, that isn't the governmental concern - what works well for SOCIETY to create the next generation of citizens. Even societies that freely accepted homosexual behavior still had marriage between a man & women, because that is what works - as a society, to raise the next generation. Government has no interest in how a man and a woman FEEL about each other, only in what they DO to raise the next generation. Laws are not made for individuals, but for how those laws affect society as a whole.
Sometime in the 60s, marriage stopped being about raising kids and started being about 'feeling love for someone'. It doesn't matter if a man feels love for his neighbor's wife - as long as he doesn't act on it. But if people feel free to leave their husband or wife, marry their neighbor's former spouse, and think it won't have a bad effect on society, they are fooling themselves. That is why marriage exists - to give approval to and promote an institution for raising kids.
Not love. Love is optional, from a societal viewpoint. Remember, for much of history, many marriages were arranged. Individuals may choose to marry 'for love', but marriage isn't supported by government 'for love'.