Because many gay men choose not to have children, they actually represent an insurance savings. When benefits where first offered, some insurance companies expected greater costs and charged a higher premium. Those costs never materialized. Presently, most insurance companies charge the same for all employees' partners. Fraud Fraud has not materialized.
Laws are already in place to cover fraudulently obtaining insurance.
Plus, the majority of people appear to be honest. Plus, signing up for benefits makes an employee responsible for their partner's medical expenses. They do not need to share income, live together, have sex, have children or even like each other. They can be in prison, they can be foreign nationals. Legal marriage does not produce or signify anything qualitative about a relationship.