ERISA pension law and yes your 401K falls under ERISA.

If You Are Married

If you are married, federal law says your spouse* is automatically the beneficiary of your 401k or other pension plan, period. You should still fill out the beneficiary form with your spouse's name, for the record.

If you want to name a beneficiary who is someone other than your spouse, your spouse must sign a waiver. The waiver MUST be in writing.

For example, you might be separated from your spouse - not divorced - and want to name a new beneficiary. Even if your intended beneficiary is a domestic partner you've been with for 20 years, your spouse will have legal claim to your 401k if you die, unless he or she signs a waiver.