Nope nope nope. Marriage isn't a right
It isn't the marriage which is the human right. It is the right not to be discriminated against on the basis of your sexuality or gender (or, in the case of mixed-race marriages in the US, race). Without same-sex marriage you have a situation where the state is saying "this couple can get married, but this couple can't because they are of the same sex."
Which is partly why same-sex marriage wasn't an issue in much of the Western world 40 years ago; discrimination on the basis of sexuality wasn't considered to be a thing - in some places same-sex relationships were still illegal. But that's progress for you.
I walk up to a pregnant woman, stab her in the gut, kill the 'foetus'. I should only be charged with assault (or possibly attempted murder).
I can't speak for most jurisdictions, but in England you couldn't be convicted of murder (or manslaughter etc.) unless the woman died. English law does not consider a foetus to be a "person", therefore it cannot be murdered.
However, if you were to cause damage to the foetus but it was still able to be born alive, then it becomes a person. If it subsequently dies as a result of your actions, that could be murder.
There is no universal definition of when life begins; there are something like 7 different points used across science and law, and ultimately someone has to make a judgement as to which definition will be used in each context.