It is a discrimination since not everyone will find somebody to marry. And even if everybody could marry, it still is a discrimination towards not married people.

Marring for the purpose to get tax benefit and oder advantages is a sign of discrimination, because they are marring to get advantages in society.

Society is full of discriminations that people have normalised and so they don’t see it as such.

That only makes sense if you assume that marriage is a purely financial thing. It has lots of other aspects that have nothing to do with money that a lot of people dislike.