Straight couples don't have to be monogamous to be married or married to be monogamous. Monogamy no more defines marriage than the presence of children does. Monogamy isn't compulsory and its absence doesn't invalidate a marriage.

That a marriage ends is less than ideal but all things end under heaven and if temporality is held to be invalidating then nothing real succeeds.

A man's friendships are like his will invalidated by marriage - but they are also no less invalidated by the marriage of his friends.

The worst thing you can do to a kid is tell them that their dreams are invalid.