And why not? Although Allstate is a large corporation, hiring and firing decisions are entirely private matters. The principle is exactly the same as if it were only a three-employee small business.
The question boils down to this: Does an employer have the right to hire and fire whom he chooses? If the answer is "Yes", then the employer's reasons for those choices are entirely immaterial. If the answer is "No", then the only remaining issue is just how far the government chooses to intrude into those decisions. The intrusions will always grow, as everything else does under government control.
The Constitution does not give the government the authority to regulate private arrangements such as employment. Doing so, even in the name of the most noble of ideals, is simply unconstitutionalm, IMO. Our government was never intended to regulate everything, making sure everyone is nice to each other. Specifially, unless an employer is practicing slavery, meaning an employee can't leave, then I don't see any standing for a complaint. If Allstate is an anti-Christian, pro-homosexual corporation, then perhaps remaining Christian employees should begin to find other jobs.
The 'intrusion' as you call it will always grow because working people will always ask the power of the state to protect them from arbitrary workplace bullying. Or to be more specific, because working women will always demand the power of the state to protect them from male bullying. It is no accident that workplace regulation is a function of the entry of women into the workforce and the very practical, realistic need of women to protect themselves. And whether newgeezer recognizes it or not the protection of women and the domestication of male urges are the prime functions of civilization.
As GOP_1900AD pointed out no contract can be valid if it is contrary to public policy. For instance, a will that mandated that the heir never marry in order to inherit would be void. A prenup demanded of a pregnant bride is void. An employment contract that allowed the employer to trample on the employee's privacy rights is similarly contrary to public policy.