马子不能代替妻媳妇才能长相依
[mă zi bù néng dài tì qī xí fù cái néng zhăng xiāng yī]
It conveys that a girlfriend (casually termed '马子') cannot replace a legitimate wife ('妻媳妇'), and true long-lasting reliance or companionship in marriage should only belong to a real spouse, suggesting a notion of traditional views towards relationship commitment.