Rich talks about two specific things in her article "Compulsory Heterosexuality". They are "lesbian existence and compulsory heterosexuality.
Lesbian existence is Rich's idea that all women live as at least partial lesbians, and almost says that women should "try out" a lesbian relationship, because it is the ultimate form of feminism. She does not necesarially mean a full out sexual relationship, though she implies it, but instead talks about how every relationship between two females is somewhere along a "scale" of lesbian-ness. My apologies for making it sound so crude. I think her first idea, the idea of trying out a "lesbian relationship", is actually a good idea. Very few people ever truly explore their sexuality, simply because society tells them that it's wrong. In reality, it can actually be a very telling experience. Whether it is something you end up wanting to remember or not, I fully believe this is something everyone should experience.
Compulsory Heterosexuality is the idea that women have been raised to firstly identify as heterosexual, because media and lineage, both good and bad, tells them that the best idea is to be straight, submissive females. Rich talks about heterosexuality as an institution, something imposed on us from birth, and this is extremely true. There are no childrens shows or popular children's books that have to do with having two mommies, and only about 15 "mainstream" books for teenagers about bisexuality and lesbianism, and how to deal with it as teenagers. I personally have read two, in one of them the girl decided she wasn't actually a lesbian, and in the other the girls cheat on each other with guys before pursuing a noncommittal sexual relationship in the woods during a summer camp. What the hell. Society needs to start putting in kid's heads that it's okay. That other people have two mommies too, and other teenagers are trying to figure out how to accept being gay too.