I dissagree, even when homosexuality was not frowned upon (say, Ancient Greece) marriage was ever reserved for man and woman, so that the rearing of children would have a legal basis and protection. Given that we acknowledge even today that (all things being equal) having both your naturlparents, together, is best the basic rational appears still to be valid.

Homosexual couples need to involve a third person (and a doctor) to create children, such children have three "parents". This has reached a point now where two Lesbians in the UK are now allowed to omit the biological father from the Birth Certificate and put down both their own names. If you want to talk about a lack logic, there is an abundence in the increasingly complex and sometimes cruel and absurd arrangementsd used to accomodate the desire of modern homosexual couples to ape heterosexual ones.

To summarise, marriage is about the children and their biological parents, ergo it cannot be extended to same-sex couples or any other irregular relationships. Even in polygamy there are multiple marriage contracts, not a group arrangement.