Not any more, it isn't.
if by modern times you mean our times, marriage is state-registered cohabitation, terminable at will by either party without fault or damages, with associated government and government-approved benefits.
Any other form of marriage is illegal.
Try making a contract, for example, not to divorce. When your wife divorces you anyway, go to court to enforce your "contract".
You will find your contract is set aside because it is "contrary to public policy". Said policy only approves state-registered cohabitation at will with benefits.
THAT'S what marriage is, in modern times.
No, of course not. You see, people everywhere will stop wanting to have children if gays have the right to marry. /sarcasm
Trace