http://www.cancer-network.org/cancer_information/lesbians_and_cancer/lesbians_and_breast_cancer.phpCigarette smoking - data suggest that lesbians smoke cigarettes at a substantially higher rate than heterosexual women.
Alcohol use - some research reports higher rates of heavy drinking among lesbians than heterosexual women
Obesity - some studies report that lesbians are more likely to be overweight or have a BMI over 25.
Pregnancy - lesbians are less likely to have biological children before age 30, which would offer some protection against cancer.
Another factor that I speculate on is the hormonal differences caused by cohabitation with another women in a sexual manner. For example, sperm is absorbed into a women's body and has a mood enhancing and hormonal effect. There is most likely chemical changes when people engage in long term sexual activity of those with the same gender and not in a positive way since that is not how the human bodies were made to work.