There is both individual and group (employer-based) health insurance. The insurance company will charge more, either way, to the individual or employer if a spouse is covered. I don't think the insurance company is concerned whether they cover more spouses (for they are getting more money) but rather about the pool of people that it bases its rates on. They try to avoid picking up the unhealthy. Unless gays are more unhealthy than the general population (some will say so, but I'm not sure it is signficant), then they should have no objection to taking on more spouses. An employer who self-funds coverage may have some concern about picking up more spouses, but if the gay population is 3% (a figure that makes sense to me) and the married gay population is some subset of this, the cost, again, may not be significant.
RYou understands this number stuff, and he may have more to add.
I used to work for an insurance company, and under their self-funded plan they covered "domestic partners". I think this is a fairly common practice amongst large corps.