well, I don't automatically think two women living together are lesbians...more suspect with men but even then, I don't automatically assume anything....
marriage offers certain benefits, like if one works, they can get their spouse on their health care plan....
There's this phenomenon in the lesbian community called "bed death" where the physical stuff ends pretty quick.
So the only thing they're doing different from lesbians is skipping out on a short honeymoon period.