and a fixed dollar amount towards the healthcare of all employees who chose to add their spouses/children/families to the insurance plan.
any employee in the 'Family' group can move to the 'Single' group
I guess you are asking about the lines of discrimination. First of all, it would be needed to find out if marital/familiar status is a protected characteristic (like race, religion or sex); discrimination is legal if it is not related to a protected characteristic. For example, it would not be illegal for your employer to provide a bonus to people who likes to collect stamps.
The discrimination laws in California define:
“protected characteristics” mean: “race, religious creed, color, national origin, ancestry, physical disability, mental disability, medical condition, marital status, sex, age, or sexual orientation”.
So yes, discrimination against married people would be illegal and could be challenged.
Now, is it discrimination? From the same page, the more related aspect is:
“Disparate impact” involves employer policies that have a disproportionate adverse effect on a protected characteristic group, e.g., a company policy of counting all absences and leaves against seniority that has a disproportionate adverse impact on women who have to take time off for pregnancy.
Now, about the issue of disparate adverse effect; yes it is more expensive if you have family that if you don't2, but:
It is your decision. You can have the exact same benefits that any single employee (individual coverage), regardless of your personal situation. Yes, your family would be without coverage but that is not your employer's responsability.
In general it is not the employer's obligation to pay to you in relation with your living costs. The mere fact that your personal circunstances mean that your living costs are higher than someone else's is not a reason to legally challenge the agreement3.
The cost of the benefit for single plans is not relevant. The fact that the cost of single plans has risen is not making your situation any worse (you are not getting reimbursed less money of the family plan than agreed). Of course, you are worse off because (probably) family plans are more expensive too, but the issue here is that the agreement did not provide any clause to recalculate the reimbursement by the employer.
To be fair, it is clear that you understand the issue, but just pointing it for future readers.
2 As a rule of thumb, almost everything is.
3 Of course, some of these individual circunstances might be covered by specific laws and agreements like the healthcare coverage here. But once those obligations are met then there is no further obligation.