There should be civil laws that enforce the marriage contract. What good are the vows, acknowledged by the government, creating a "legal" marriage if the parties are not subject to a penalty for not keeping their word? The alternative is to do away with government permitted marriage and just allow people to be married without a marriage license.
Secular marriage is a contract ... granted, a very specialized contract but a contract all the same ... because marriage almost always means the individuals' finances are wholly co-mingled, there needs to be specific rules for dividing up the money when the couple decides to dissolve their contract ... and this is different than cohabitating, just simply living together, mingling the finances and hope for the best ...
Certainly, we can outlaw sex if the purpose isn't procreation ... if we could effectively enforce such a law, abortion rates would crash to almost nothing ... next we could limit haircut styles for men to two or three ... make women stop wearing pants ... force everyone to politely wait at 4-way stops ... the slippery slope ...